import asyncio
from datetime import datetime
from typing import Dict, List, Optional

from sqlalchemy import and_, delete, func, or_, select
from sqlalchemy.ext.asyncio import AsyncSession

from letta.errors import LettaInvalidArgumentError
from letta.log import get_logger
from letta.orm.agent import Agent as AgentModel
from letta.orm.block import Block as BlockModel
from letta.orm.block_history import BlockHistory
from letta.orm.blocks_agents import BlocksAgents
from letta.orm.errors import NoResultFound
from letta.otel.tracing import trace_method
from letta.schemas.agent import AgentState as PydanticAgentState
from letta.schemas.block import Block as PydanticBlock, BlockUpdate
from letta.schemas.enums import ActorType, PrimitiveType
from letta.schemas.user import User as PydanticUser
from letta.server.db import db_registry
from letta.settings import DatabaseChoice, settings
from letta.utils import bounded_gather, decrypt_agent_secrets, enforce_types
from letta.validators import raise_on_invalid_id

logger = get_logger(__name__)


def validate_block_limit_constraint(update_data: dict, existing_block: BlockModel) -> None:
    """
    Validates that block limit constraints are satisfied when updating a block.

    Rules:
    - If limit is being updated, it must be >= the length of the value (existing or new)
    - If value is being updated, its length must not exceed the limit (existing or new)

    Args:
        update_data: Dictionary of fields to update
        existing_block: The current block being updated

    Raises:
        LettaInvalidArgumentError: If validation fails
    """
    # If limit is being updated, ensure it's >= current value length
    if "limit" in update_data:
        # Get the value that will be used (either from update_data or existing)
        value_to_check = update_data.get("value", existing_block.value)
        limit_to_check = update_data["limit"]
        if value_to_check and limit_to_check < len(value_to_check):
            raise LettaInvalidArgumentError(
                f"Limit ({limit_to_check}) cannot be less than current value length ({len(value_to_check)} characters)",
                argument_name="limit",
            )
    # If value is being updated and there's an existing limit, ensure value doesn't exceed limit
    elif "value" in update_data and existing_block.limit:
        if len(update_data["value"]) > existing_block.limit:
            raise LettaInvalidArgumentError(
                f"Value length ({len(update_data['value'])} characters) exceeds block limit ({existing_block.limit} characters)",
                argument_name="value",
            )


def validate_block_creation(block_data: dict) -> None:
    """
    Validates that block limit constraints are satisfied when creating a block.

    Rules:
    - If both value and limit are provided, limit must be >= value length

    Args:
        block_data: Dictionary of block fields for creation

    Raises:
        LettaInvalidArgumentError: If validation fails
    """
    value = block_data.get("value")
    limit = block_data.get("limit")

    if value and limit and len(value) > limit:
        raise LettaInvalidArgumentError(
            f"Block limit ({limit}) must be greater than or equal to value length ({len(value)} characters)", argument_name="limit"
        )


class BlockManager:
    """Manager class to handle business logic related to Blocks."""

    @enforce_types
    @trace_method
    async def create_or_update_block_async(self, block: PydanticBlock, actor: PydanticUser) -> PydanticBlock:
        """Create a new block based on the Block schema."""
        db_block = await self.get_block_by_id_async(block.id, actor)
        if db_block:
            update_data = BlockUpdate(**block.model_dump(to_orm=True, exclude_none=True))
            return await self.update_block_async(block.id, update_data, actor)
        else:
            async with db_registry.async_session() as session:
                data = block.model_dump(to_orm=True, exclude_none=True)
                # Validate block creation constraints
                validate_block_creation(data)
                block = BlockModel(**data, organization_id=actor.organization_id)
                await block.create_async(session, actor=actor, no_commit=True, no_refresh=True)
                pydantic_block = block.to_pydantic()
                # context manager now handles commits
                # await session.commit()
                return pydantic_block

    @enforce_types
    @trace_method
    async def batch_create_blocks_async(self, blocks: List[PydanticBlock], actor: PydanticUser) -> List[PydanticBlock]:
        """
        Batch-create multiple Blocks in one transaction for better performance.
        Args:
            blocks: List of PydanticBlock schemas to create
            actor:    The user performing the operation
        Returns:
            List of created PydanticBlock instances (with IDs, timestamps, etc.)
        """
        if not blocks:
            return []

        async with db_registry.async_session() as session:
            # Validate all blocks before creating any
            validated_data = []
            for block in blocks:
                block_data = block.model_dump(to_orm=True, exclude_none=True)
                validate_block_creation(block_data)
                validated_data.append(block_data)

            block_models = [BlockModel(**data, organization_id=actor.organization_id) for data in validated_data]
            created_models = await BlockModel.batch_create_async(
                items=block_models, db_session=session, actor=actor, no_commit=True, no_refresh=True
            )
            result = [m.to_pydantic() for m in created_models]
            # context manager now handles commits
            # await session.commit()
            return result

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def update_block_async(self, block_id: str, block_update: BlockUpdate, actor: PydanticUser) -> PydanticBlock:
        """Update a block by its ID with the given BlockUpdate object."""
        async with db_registry.async_session() as session:
            block = await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)
            update_data = block_update.model_dump(to_orm=True, exclude_unset=True, exclude_none=True)

            # Validate limit constraints before updating
            validate_block_limit_constraint(update_data, block)

            for key, value in update_data.items():
                setattr(block, key, value)

            await block.update_async(db_session=session, actor=actor, no_commit=True, no_refresh=True)
            pydantic_block = block.to_pydantic()
            # context manager now handles commits
            # await session.commit()
            return pydantic_block

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def delete_block_async(self, block_id: str, actor: PydanticUser) -> None:
        """Delete a block by its ID."""
        async with db_registry.async_session() as session:
            # First, delete all references in blocks_agents table
            await session.execute(delete(BlocksAgents).where(BlocksAgents.block_id == block_id))
            await session.flush()

            # Then delete the block itself
            block = await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)
            await block.hard_delete_async(db_session=session, actor=actor)

    @enforce_types
    @trace_method
    async def get_blocks_async(
        self,
        actor: PydanticUser,
        label: Optional[str] = None,
        is_template: Optional[bool] = None,
        template_name: Optional[str] = None,
        identity_id: Optional[str] = None,
        identifier_keys: Optional[List[str]] = None,
        project_id: Optional[str] = None,
        before: Optional[str] = None,
        after: Optional[str] = None,
        limit: Optional[int] = 50,
        label_search: Optional[str] = None,
        description_search: Optional[str] = None,
        value_search: Optional[str] = None,
        connected_to_agents_count_gt: Optional[int] = None,
        connected_to_agents_count_lt: Optional[int] = None,
        connected_to_agents_count_eq: Optional[List[int]] = None,
        ascending: bool = True,
        show_hidden_blocks: Optional[bool] = None,
    ) -> List[PydanticBlock]:
        """Async version of get_blocks method. Retrieve blocks based on various optional filters."""
        from sqlalchemy import select
        from sqlalchemy.orm import noload

        from letta.orm.sqlalchemy_base import AccessType

        async with db_registry.async_session() as session:
            # Start with a basic query
            query = select(BlockModel)

            # Explicitly avoid loading relationships
            query = query.options(noload(BlockModel.agents), noload(BlockModel.identities), noload(BlockModel.groups))

            # Apply access control
            query = BlockModel.apply_access_predicate(query, actor, ["read"], AccessType.ORGANIZATION)

            # Add filters
            query = query.where(BlockModel.organization_id == actor.organization_id)
            if label:
                query = query.where(BlockModel.label == label)

            if is_template is not None:
                query = query.where(BlockModel.is_template == is_template)

            if template_name:
                query = query.where(BlockModel.template_name == template_name)

            if project_id:
                query = query.where(BlockModel.project_id == project_id)

            if label_search and not label:
                query = query.where(BlockModel.label.ilike(f"%{label_search}%"))

            if description_search:
                query = query.where(BlockModel.description.ilike(f"%{description_search}%"))

            if value_search:
                query = query.where(BlockModel.value.ilike(f"%{value_search}%"))

            # Apply hidden filter
            if not show_hidden_blocks:
                query = query.where((BlockModel.hidden.is_(None)) | (BlockModel.hidden == False))

            needs_distinct = False

            needs_agent_count_join = any(
                condition is not None
                for condition in [connected_to_agents_count_gt, connected_to_agents_count_lt, connected_to_agents_count_eq]
            )

            # If any agent count filters are specified, create a single subquery and apply all filters
            if needs_agent_count_join:
                # Create a subquery to count agents per block
                agent_count_subquery = (
                    select(BlocksAgents.block_id, func.count(BlocksAgents.agent_id).label("agent_count"))
                    .group_by(BlocksAgents.block_id)
                    .subquery()
                )

                # Determine if we need a left join (for cases involving 0 counts)
                needs_left_join = (connected_to_agents_count_lt is not None) or (
                    connected_to_agents_count_eq is not None and 0 in connected_to_agents_count_eq
                )

                if needs_left_join:
                    # Left join to include blocks with no agents
                    query = query.outerjoin(agent_count_subquery, BlockModel.id == agent_count_subquery.c.block_id)
                    # Use coalesce to treat NULL as 0 for blocks with no agents
                    agent_count_expr = func.coalesce(agent_count_subquery.c.agent_count, 0)
                else:
                    # Inner join since we don't need blocks with no agents
                    query = query.join(agent_count_subquery, BlockModel.id == agent_count_subquery.c.block_id)
                    agent_count_expr = agent_count_subquery.c.agent_count

                # Build the combined filter conditions
                conditions = []

                if connected_to_agents_count_gt is not None:
                    conditions.append(agent_count_expr > connected_to_agents_count_gt)

                if connected_to_agents_count_lt is not None:
                    conditions.append(agent_count_expr < connected_to_agents_count_lt)

                if connected_to_agents_count_eq is not None:
                    conditions.append(agent_count_expr.in_(connected_to_agents_count_eq))

                # Apply all conditions with AND logic
                if conditions:
                    query = query.where(and_(*conditions))

                needs_distinct = True

            if identifier_keys:
                query = query.join(BlockModel.identities).filter(
                    BlockModel.identities.property.mapper.class_.identifier_key.in_(identifier_keys)
                )
                needs_distinct = True

            if identity_id:
                query = query.join(BlockModel.identities).filter(BlockModel.identities.property.mapper.class_.id == identity_id)
                needs_distinct = True

            if after:
                result = (await session.execute(select(BlockModel.created_at, BlockModel.id).where(BlockModel.id == after))).first()
                if result:
                    after_sort_value, after_id = result
                    # SQLite does not support as granular timestamping, so we need to round the timestamp
                    if settings.database_engine is DatabaseChoice.SQLITE and isinstance(after_sort_value, datetime):
                        after_sort_value = after_sort_value.strftime("%Y-%m-%d %H:%M:%S")

                    if ascending:
                        query = query.where(
                            BlockModel.created_at > after_sort_value,
                            or_(BlockModel.created_at == after_sort_value, BlockModel.id > after_id),
                        )
                    else:
                        query = query.where(
                            BlockModel.created_at < after_sort_value,
                            or_(BlockModel.created_at == after_sort_value, BlockModel.id < after_id),
                        )

            if before:
                result = (await session.execute(select(BlockModel.created_at, BlockModel.id).where(BlockModel.id == before))).first()
                if result:
                    before_sort_value, before_id = result
                    # SQLite does not support as granular timestamping, so we need to round the timestamp
                    if settings.database_engine is DatabaseChoice.SQLITE and isinstance(before_sort_value, datetime):
                        before_sort_value = before_sort_value.strftime("%Y-%m-%d %H:%M:%S")

                    if ascending:
                        query = query.where(
                            BlockModel.created_at < before_sort_value,
                            or_(BlockModel.created_at == before_sort_value, BlockModel.id < before_id),
                        )
                    else:
                        query = query.where(
                            BlockModel.created_at > before_sort_value,
                            or_(BlockModel.created_at == before_sort_value, BlockModel.id > before_id),
                        )

            # Apply ordering and handle distinct if needed
            if needs_distinct:
                if ascending:
                    query = query.distinct(BlockModel.id).order_by(BlockModel.id.asc(), BlockModel.created_at.asc())
                else:
                    query = query.distinct(BlockModel.id).order_by(BlockModel.id.desc(), BlockModel.created_at.desc())
            else:
                if ascending:
                    query = query.order_by(BlockModel.created_at.asc(), BlockModel.id.asc())
                else:
                    query = query.order_by(BlockModel.created_at.desc(), BlockModel.id.desc())

            # Add limit
            if limit:
                query = query.limit(limit)

            # Execute the query
            result = await session.execute(query)
            blocks = result.scalars().all()

            return [block.to_pydantic() for block in blocks]

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def get_block_by_id_async(self, block_id: str, actor: Optional[PydanticUser] = None) -> Optional[PydanticBlock]:
        """Retrieve a block by its name."""
        async with db_registry.async_session() as session:
            try:
                block = await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)
                return block.to_pydantic()
            except NoResultFound:
                return None

    @enforce_types
    @trace_method
    async def get_all_blocks_by_ids_async(self, block_ids: List[str], actor: Optional[PydanticUser] = None) -> List[PydanticBlock]:
        """Retrieve blocks by their ids without loading unnecessary relationships. Async implementation."""
        from sqlalchemy import select
        from sqlalchemy.orm import noload

        from letta.orm.sqlalchemy_base import AccessType

        if not block_ids:
            return []

        async with db_registry.async_session() as session:
            # Start with a basic query
            query = select(BlockModel)

            # Add ID filter
            query = query.where(BlockModel.id.in_(block_ids))

            # Explicitly avoid loading relationships
            query = query.options(noload(BlockModel.agents), noload(BlockModel.identities), noload(BlockModel.groups))

            # Apply access control if actor is provided
            if actor:
                query = BlockModel.apply_access_predicate(query, actor, ["read"], AccessType.ORGANIZATION)

            # TODO: Add soft delete filter if applicable
            # if hasattr(BlockModel, "is_deleted"):
            #     query = query.where(BlockModel.is_deleted == False)

            # Execute the query
            result = await session.execute(query)
            blocks = result.scalars().all()

            # Convert to Pydantic models
            pydantic_blocks = [block.to_pydantic() for block in blocks]

            # For backward compatibility, add None for missing blocks
            if len(pydantic_blocks) < len(block_ids):
                {block.id for block in pydantic_blocks}
                result_blocks = []
                for block_id in block_ids:
                    block = next((b for b in pydantic_blocks if b.id == block_id), None)
                    result_blocks.append(block)
                return result_blocks

            return pydantic_blocks

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def get_agents_for_block_async(
        self,
        block_id: str,
        actor: PydanticUser,
        include_relationships: Optional[List[str]] = None,
        include: List[str] = [],
        before: Optional[str] = None,
        after: Optional[str] = None,
        limit: Optional[int] = 50,
        ascending: bool = True,
    ) -> List[PydanticAgentState]:
        """
        Retrieve all agents associated with a given block with pagination support.

        Args:
            block_id: ID of the block to get agents for
            actor: User performing the operation
            include_relationships: List of relationships to include in the response
            before: Cursor for pagination (get items before this ID)
            after: Cursor for pagination (get items after this ID)
            limit: Maximum number of items to return
            ascending: Sort order (True for ascending, False for descending)

        Returns:
            List of agent states associated with the block
        """
        async with db_registry.async_session() as session:
            # Start with a basic query
            query = (
                select(AgentModel)
                .where(AgentModel.id.in_(select(BlocksAgents.agent_id).where(BlocksAgents.block_id == block_id)))
                .where(AgentModel.organization_id == actor.organization_id)
            )

            # Apply pagination using cursor-based approach
            if after:
                result = (await session.execute(select(AgentModel.created_at, AgentModel.id).where(AgentModel.id == after))).first()
                if result:
                    after_sort_value, after_id = result
                    # SQLite does not support as granular timestamping, so we need to round the timestamp
                    if settings.database_engine is DatabaseChoice.SQLITE and isinstance(after_sort_value, datetime):
                        after_sort_value = after_sort_value.strftime("%Y-%m-%d %H:%M:%S")

                    if ascending:
                        query = query.where(
                            AgentModel.created_at > after_sort_value,
                            or_(AgentModel.created_at == after_sort_value, AgentModel.id > after_id),
                        )
                    else:
                        query = query.where(
                            AgentModel.created_at < after_sort_value,
                            or_(AgentModel.created_at == after_sort_value, AgentModel.id < after_id),
                        )

            if before:
                result = (await session.execute(select(AgentModel.created_at, AgentModel.id).where(AgentModel.id == before))).first()
                if result:
                    before_sort_value, before_id = result
                    # SQLite does not support as granular timestamping, so we need to round the timestamp
                    if settings.database_engine is DatabaseChoice.SQLITE and isinstance(before_sort_value, datetime):
                        before_sort_value = before_sort_value.strftime("%Y-%m-%d %H:%M:%S")

                    if ascending:
                        query = query.where(
                            AgentModel.created_at < before_sort_value,
                            or_(AgentModel.created_at == before_sort_value, AgentModel.id < before_id),
                        )
                    else:
                        query = query.where(
                            AgentModel.created_at > before_sort_value,
                            or_(AgentModel.created_at == before_sort_value, AgentModel.id > before_id),
                        )

            # Apply sorting
            if ascending:
                query = query.order_by(AgentModel.created_at.asc(), AgentModel.id.asc())
            else:
                query = query.order_by(AgentModel.created_at.desc(), AgentModel.id.desc())

            # Apply limit
            if limit:
                query = query.limit(limit)

            # Execute the query
            result = await session.execute(query)
            agents_orm = result.scalars().all()

            # Convert without decrypting to release DB connection before PBKDF2
            agents_encrypted = await bounded_gather(
                [agent.to_pydantic_async(include_relationships=[], include=include, decrypt=False) for agent in agents_orm]
            )

        # Decrypt secrets outside session
        return await decrypt_agent_secrets(agents_encrypted)

    @enforce_types
    @trace_method
    async def size_async(self, actor: PydanticUser) -> int:
        """
        Get the total count of blocks for the given user.
        """
        async with db_registry.async_session() as session:
            return await BlockModel.size_async(db_session=session, actor=actor)

    # Block History Functions

    @enforce_types
    async def _move_block_to_sequence(self, session: AsyncSession, block: BlockModel, target_seq: int, actor: PydanticUser) -> BlockModel:
        """
        Internal helper that moves the 'block' to the specified 'target_seq' within BlockHistory.
        1) Find the BlockHistory row at sequence_number=target_seq
        2) Copy fields into the block
        3) Update and flush (no_commit=True) - the caller is responsible for final commit

        Raises:
            NoResultFound: if no BlockHistory row for (block_id, target_seq)
        """
        if not block.id:
            raise ValueError("Block is missing an ID. Cannot move sequence.")

        stmt = select(BlockHistory).filter(
            BlockHistory.block_id == block.id,
            BlockHistory.sequence_number == target_seq,
        )
        result = await session.execute(stmt)
        target_entry = result.scalar_one_or_none()
        if not target_entry:
            raise NoResultFound(f"No BlockHistory row found for block_id={block.id} at sequence={target_seq}")

        # Copy fields from target_entry to block
        block.description = target_entry.description  # type: ignore
        block.label = target_entry.label  # type: ignore
        block.value = target_entry.value  # type: ignore
        block.limit = target_entry.limit  # type: ignore
        block.metadata_ = target_entry.metadata_  # type: ignore
        block.current_history_entry_id = target_entry.id  # type: ignore

        # Update in DB (optimistic locking).
        # We'll do a flush now; the caller does final commit.
        updated_block = await block.update_async(db_session=session, actor=actor, no_commit=True)
        return updated_block

    @enforce_types
    @trace_method
    async def bulk_update_block_values_async(
        self, updates: Dict[str, str], actor: PydanticUser, return_hydrated: bool = False
    ) -> Optional[List[PydanticBlock]]:
        """
        Bulk-update the `value` field for multiple blocks in one transaction.

        Args:
            updates: mapping of block_id -> new value
            actor:   the user performing the update (for org scoping, permissions, audit)
            return_hydrated: whether to return the pydantic Block objects that were updated

        Returns:
            the updated Block objects as Pydantic schemas

        Raises:
            NoResultFound if any block_id doesn't exist or isn't visible to this actor
            ValueError     if any new value exceeds its block's limit
        """
        async with db_registry.async_session() as session:
            query = select(BlockModel).where(BlockModel.id.in_(updates.keys()), BlockModel.organization_id == actor.organization_id)
            result = await session.execute(query)
            blocks = result.scalars().all()

            found_ids = {b.id for b in blocks}
            missing = set(updates.keys()) - found_ids
            if missing:
                logger.warning(f"Block IDs not found or inaccessible, skipping during bulk update: {missing!r}")

            for block in blocks:
                new_val = updates[block.id]
                if len(new_val) > block.limit:
                    logger.warning(f"Value length ({len(new_val)}) exceeds limit ({block.limit}) for block {block.id!r}, truncating...")
                    new_val = new_val[: block.limit]
                block.value = new_val

            # context manager now handles commits
            # await session.commit()

            if return_hydrated:
                # TODO: implement for async
                pass

            return None

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @raise_on_invalid_id(param_name="agent_id", expected_prefix=PrimitiveType.AGENT)
    @trace_method
    async def checkpoint_block_async(
        self,
        block_id: str,
        actor: PydanticUser,
        agent_id: Optional[str] = None,
        use_preloaded_block: Optional[BlockModel] = None,  # For concurrency tests
    ) -> PydanticBlock:
        """
        Create a new checkpoint for the given Block by copying its
        current state into BlockHistory, using SQLAlchemy's built-in
        version_id_col for concurrency checks.

        - If the block was undone to an earlier checkpoint, we remove
          any "future" checkpoints beyond the current state to keep a
          strictly linear history.
        - A single commit at the end ensures atomicity.
        """
        async with db_registry.async_session() as session:
            # 1) Load the Block
            if use_preloaded_block is not None:
                block = await session.merge(use_preloaded_block)
            else:
                block = await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)

            # 2) Identify the block's current checkpoint (if any)
            current_entry = None
            if block.current_history_entry_id:
                current_entry = await session.get(BlockHistory, block.current_history_entry_id)

            # The current sequence, or 0 if no checkpoints exist
            current_seq = current_entry.sequence_number if current_entry else 0

            # 3) Truncate any future checkpoints
            #    If we are at seq=2, but there's a seq=3 or higher from a prior "redo chain",
            #    remove those, so we maintain a strictly linear undo/redo stack.
            stmt = select(BlockHistory).filter(BlockHistory.block_id == block.id, BlockHistory.sequence_number > current_seq)
            result = await session.execute(stmt)
            for entry in result.scalars():
                session.delete(entry)

            # Flush the deletes to ensure they're executed before we create a new entry
            await session.flush()

            # 4) Determine the next sequence number
            next_seq = current_seq + 1

            # 5) Create a new BlockHistory row reflecting the block's current state
            history_entry = BlockHistory(
                organization_id=actor.organization_id,
                block_id=block.id,
                sequence_number=next_seq,
                description=block.description,
                label=block.label,
                value=block.value,
                limit=block.limit,
                metadata_=block.metadata_,
                actor_type=ActorType.LETTA_AGENT if agent_id else ActorType.LETTA_USER,
                actor_id=agent_id if agent_id else actor.id,
            )
            await history_entry.create_async(session, actor=actor, no_commit=True)

            # 6) Update the block’s pointer to the new checkpoint
            block.current_history_entry_id = history_entry.id

            # 7) Flush changes, then commit once
            block = await block.update_async(db_session=session, actor=actor, no_commit=True)
            # context manager now handles commits
            # await session.commit()

            return block.to_pydantic()

    @enforce_types
    async def _move_block_to_sequence(self, session: AsyncSession, block: BlockModel, target_seq: int, actor: PydanticUser) -> BlockModel:
        """
        Internal helper that moves the 'block' to the specified 'target_seq' within BlockHistory.
        1) Find the BlockHistory row at sequence_number=target_seq
        2) Copy fields into the block
        3) Update and flush (no_commit=True) - the caller is responsible for final commit

        Raises:
            NoResultFound: if no BlockHistory row for (block_id, target_seq)
        """
        if not block.id:
            raise ValueError("Block is missing an ID. Cannot move sequence.")

        stmt = select(BlockHistory).filter(
            BlockHistory.block_id == block.id,
            BlockHistory.sequence_number == target_seq,
        )
        result = await session.execute(stmt)
        target_entry = result.scalar_one_or_none()
        if not target_entry:
            raise NoResultFound(f"No BlockHistory row found for block_id={block.id} at sequence={target_seq}")

        # Copy fields from target_entry to block
        block.description = target_entry.description  # type: ignore
        block.label = target_entry.label  # type: ignore
        block.value = target_entry.value  # type: ignore
        block.limit = target_entry.limit  # type: ignore
        block.metadata_ = target_entry.metadata_  # type: ignore
        block.current_history_entry_id = target_entry.id  # type: ignore

        # Update in DB (optimistic locking).
        # We'll do a flush now; the caller does final commit.
        updated_block = await block.update_async(db_session=session, actor=actor, no_commit=True)
        return updated_block

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def undo_checkpoint_block(
        self, block_id: str, actor: PydanticUser, use_preloaded_block: Optional[BlockModel] = None
    ) -> PydanticBlock:
        """
        Move the block to the immediately previous checkpoint in BlockHistory.
        If older sequences have been pruned, we jump to the largest sequence
        number that is still < current_seq.
        """
        async with db_registry.async_session() as session:
            # 1) Load the current block
            block = (
                await session.merge(use_preloaded_block)
                if use_preloaded_block
                else await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)
            )

            if not block.current_history_entry_id:
                raise LettaInvalidArgumentError(f"Block {block_id} has no history entry - cannot undo.", argument_name="block_id")

            current_entry = await session.get(BlockHistory, block.current_history_entry_id)
            if not current_entry:
                raise NoResultFound(f"BlockHistory row not found for id={block.current_history_entry_id}")

            current_seq = current_entry.sequence_number

            # 2) Find the largest sequence < current_seq
            stmt = (
                select(BlockHistory)
                .filter(BlockHistory.block_id == block.id, BlockHistory.sequence_number < current_seq)
                .order_by(BlockHistory.sequence_number.desc())
                .limit(1)
            )
            result = await session.execute(stmt)
            previous_entry = result.scalar_one_or_none()
            if not previous_entry:
                # No earlier checkpoint available
                raise LettaInvalidArgumentError(
                    f"Block {block_id} is already at the earliest checkpoint (seq={current_seq}). Cannot undo further.",
                    argument_name="block_id",
                )

            # 3) Move to that sequence
            block = await self._move_block_to_sequence(session, block, previous_entry.sequence_number, actor)

            # 4) Commit
            # context manager now handles commits
            # await session.commit()
            return block.to_pydantic()

    @enforce_types
    @raise_on_invalid_id(param_name="block_id", expected_prefix=PrimitiveType.BLOCK)
    @trace_method
    async def redo_checkpoint_block(
        self, block_id: str, actor: PydanticUser, use_preloaded_block: Optional[BlockModel] = None
    ) -> PydanticBlock:
        """
        Move the block to the next checkpoint if it exists.
        If some middle checkpoints have been pruned, we jump to the smallest
        sequence > current_seq that remains.
        """
        async with db_registry.async_session() as session:
            block = (
                await session.merge(use_preloaded_block)
                if use_preloaded_block
                else await BlockModel.read_async(db_session=session, identifier=block_id, actor=actor)
            )

            if not block.current_history_entry_id:
                raise LettaInvalidArgumentError(f"Block {block_id} has no history entry - cannot redo.", argument_name="block_id")

            current_entry = await session.get(BlockHistory, block.current_history_entry_id)
            if not current_entry:
                raise LettaInvalidArgumentError(
                    f"BlockHistory row not found for id={block.current_history_entry_id}", argument_name="block_id"
                )

            current_seq = current_entry.sequence_number

            # Find the smallest sequence that is > current_seq
            stmt = (
                select(BlockHistory)
                .filter(BlockHistory.block_id == block.id, BlockHistory.sequence_number > current_seq)
                .order_by(BlockHistory.sequence_number.asc())
                .limit(1)
            )
            result = await session.execute(stmt)
            next_entry = result.scalar_one_or_none()
            if not next_entry:
                raise LettaInvalidArgumentError(
                    f"Block {block_id} is at the highest checkpoint (seq={current_seq}). Cannot redo further.", argument_name="block_id"
                )

            block = await self._move_block_to_sequence(session, block, next_entry.sequence_number, actor)

            # context manager now handles commits
            # await session.commit()
            return block.to_pydantic()
