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FastAPI Core Service

Overview

This skill covers creating the base service class that acts as an intermediary between routers and repositories. Services contain business logic and orchestrate repository calls.

Create core/service.py

Create src/app/core/service.py:

from collections.abc import Sequence
from typing import Generic
from uuid import UUID

from fastapi_filter.contrib.sqlalchemy import Filter
from fastapi_pagination import Params
from fastapi_pagination.bases import AbstractPage

from app.core.repository import (
    AbstractRepository,
    CreateSchemaType,
    ModelType,
    UpdateSchemaType,
)


class BaseService(Generic[ModelType, CreateSchemaType, UpdateSchemaType]):
    """
    Base service class providing standard CRUD operations.
    
    Services:
    - Wrap repository operations
    - Contain business logic
    - Orchestrate multiple repository calls
    - Handle cross-cutting concerns (logging, events, etc.)
    
    Services should NOT:
    - Write SQL queries (delegate to repository)
    - Handle HTTP concerns (that's the router's job)
    - Access the database session directly
    
    Type Parameters:
        ModelType: SQLAlchemy model class
        CreateSchemaType: Pydantic schema for create operations
        UpdateSchemaType: Pydantic schema for update operations
    
    Usage:
        class ItemService(BaseService[Item, ItemCreate, ItemUpdate]):
            pass
    """

    def __init__(
        self,
        repository: AbstractRepository[ModelType, CreateSchemaType, UpdateSchemaType],
    ):
        """
        Initialize service with repository.
        
        Args:
            repository: Repository instance for data access
        """
        self._repository = repository

    # ==================== Basic CRUD ====================

    async def create(self, obj_in: CreateSchemaType) -> ModelType:
        """
        Create a new record.
        
        Override to add business logic before/after creation.
        
        Args:
            obj_in: Creation data
            
        Returns:
            Created model instance
        """
        return await self._repository.create(obj_in)

    async def get_by_id(self, id: UUID) -> ModelType | None:
        """
        Get a single record by ID.
        
        Args:
            id: UUID of the record
            
        Returns:
            Model instance if found, None otherwise
        """
        return await self._repository.get_by_id(id)

    async def get_all(self) -> Sequence[ModelType]:
        """
        Get all records.
        
        Returns:
            Sequence of model instances
        """
        return await self._repository.get_all()

    async def update(
        self,
        id: UUID,
        obj_in: UpdateSchemaType,
        exclude_unset: bool = True,
    ) -> ModelType | None:
        """
        Update an existing record.
        
        Override to add business logic before/after update.
        
        Args:
            id: UUID of record to update
            obj_in: Update data
            exclude_unset: Only update explicitly set fields
            
        Returns:
            Updated model instance if found, None otherwise
        """
        return await self._repository.update(id, obj_in, exclude_unset)

    async def delete(self, id: UUID) -> bool:
        """
        Permanently delete a record.
        
        Args:
            id: UUID of record to delete
            
        Returns:
            True if deleted, False if not found
        """
        return await self._repository.delete(id)

    # ==================== Pagination & Filtering ====================

    async def get_paginated(
        self,
        params: Params,
        filter_spec: Filter | None = None,
    ) -> AbstractPage[ModelType]:
        """
        Get paginated records with optional filtering.
        
        This method receives the filter from the router layer and
        passes it to the repository.
        
        Args:
            params: Pagination parameters
            filter_spec: Optional filter specification
            
        Returns:
            Paginated result
        """
        return await self._repository.get_paginated(params, filter_spec)

    async def get_filtered(
        self,
        filter_spec: Filter,
    ) -> Sequence[ModelType]:
        """
        Get all records matching filter.
        
        Args:
            filter_spec: Filter specification
            
        Returns:
            Matching records
        """
        return await self._repository.get_filtered(filter_spec)

    async def count(self, filter_spec: Filter | None = None) -> int:
        """
        Count records.
        
        Args:
            filter_spec: Optional filter specification
            
        Returns:
            Number of matching records
        """
        return await self._repository.count(filter_spec)

    # ==================== Bulk Operations ====================

    async def bulk_create(
        self,
        objs_in: Sequence[CreateSchemaType],
    ) -> Sequence[ModelType]:
        """
        Create multiple records.
        
        Args:
            objs_in: Sequence of creation data
            
        Returns:
            Created model instances
        """
        return await self._repository.bulk_create(objs_in)

    async def bulk_upsert(
        self,
        objs_in: Sequence[CreateSchemaType],
        index_elements: Sequence[str],
        update_fields: Sequence[str] | None = None,
    ) -> Sequence[ModelType]:
        """
        Upsert multiple records.
        
        Args:
            objs_in: Records to upsert
            index_elements: Unique constraint columns
            update_fields: Fields to update on conflict
            
        Returns:
            Upserted model instances
        """
        return await self._repository.bulk_upsert(
            objs_in, index_elements, update_fields
        )

    async def bulk_delete(self, ids: Sequence[UUID]) -> int:
        """
        Delete multiple records.
        
        Args:
            ids: UUIDs to delete
            
        Returns:
            Number deleted
        """
        return await self._repository.bulk_delete(ids)

    # ==================== Soft Delete ====================

    async def soft_delete(self, id: UUID) -> bool:
        """
        Soft delete a record.
        
        Args:
            id: UUID of record
            
        Returns:
            True if soft deleted
        """
        return await self._repository.soft_delete(id)

    async def restore(self, id: UUID) -> bool:
        """
        Restore a soft-deleted record.
        
        Args:
            id: UUID of record
            
        Returns:
            True if restored
        """
        return await self._repository.restore(id)

    async def get_by_id_with_deleted(self, id: UUID) -> ModelType | None:
        """
        Get record including soft-deleted.
        
        Args:
            id: UUID of record
            
        Returns:
            Model instance if found
        """
        return await self._repository.get_by_id_with_deleted(id)

    # ==================== Utility ====================

    async def exists(self, id: UUID) -> bool:
        """
        Check if record exists.
        
        Args:
            id: UUID to check
            
        Returns:
            True if exists
        """
        return await self._repository.exists(id)

    async def get_by_ids(self, ids: Sequence[UUID]) -> Sequence[ModelType]:
        """
        Get multiple records by IDs.
        
        Args:
            ids: UUIDs to fetch
            
        Returns:
            Found model instances
        """
        return await self._repository.get_by_ids(ids)

Usage: Creating Entity Services

# src/app/items/service.py
from uuid import UUID

from app.core.service import BaseService
from app.exceptions import ConflictError, NotFoundError
from app.items.models import Item
from app.items.repository import ItemRepository
from app.items.schemas import ItemCreate, ItemUpdate


class ItemService(BaseService[Item, ItemCreate, ItemUpdate]):
    """Service for Item entity with business logic."""

    def __init__(self, repository: ItemRepository):
        super().__init__(repository)
        # Type hint for IDE support
        self._repository: ItemRepository = repository

    async def create(self, obj_in: ItemCreate) -> Item:
        """
        Create item with duplicate name check.
        
        Raises:
            ConflictError: If item with same name exists
        """
        # Business logic: check for duplicate name
        existing = await self._repository.get_by_name(obj_in.name)
        if existing:
            raise ConflictError(
                resource="Item",
                field="name",
                value=obj_in.name,
            )

        return await super().create(obj_in)

    async def get_by_id_or_raise(self, id: UUID) -> Item:
        """
        Get item by ID or raise NotFoundError.
        
        Useful when you need to ensure the item exists.
        
        Raises:
            NotFoundError: If item not found
        """
        item = await self.get_by_id(id)
        if not item:
            raise NotFoundError(resource="Item", id=id)
        return item

    async def update(
        self,
        id: UUID,
        obj_in: ItemUpdate,
        exclude_unset: bool = True,
    ) -> Item | None:
        """
        Update item with duplicate name check.
        
        Raises:
            ConflictError: If new name conflicts with existing item
        """
        # Business logic: check name uniqueness on update
        if obj_in.name is not None:
            existing = await self._repository.get_by_name(obj_in.name)
            if existing and existing.id != id:
                raise ConflictError(
                    resource="Item",
                    field="name",
                    value=obj_in.name,
                )

        return await super().update(id, obj_in, exclude_unset)

Service Patterns

1. Get or Raise Pattern

async def get_by_id_or_raise(self, id: UUID) -> ModelType:
    """Get record or raise NotFoundError."""
    instance = await self.get_by_id(id)
    if not instance:
        raise NotFoundError(resource=self._model_name, id=id)
    return instance

2. Validation Before Create

async def create(self, obj_in: CreateSchemaType) -> ModelType:
    """Create with pre-validation."""
    await self._validate_create(obj_in)
    return await super().create(obj_in)

async def _validate_create(self, obj_in: CreateSchemaType) -> None:
    """Override in subclass to add validation logic."""
    pass

3. Multi-Repository Orchestration

class OrderService(BaseService[Order, OrderCreate, OrderUpdate]):
    def __init__(
        self,
        repository: OrderRepository,
        item_repository: ItemRepository,
        user_repository: UserRepository,
    ):
        super().__init__(repository)
        self._item_repo = item_repository
        self._user_repo = user_repository

    async def create(self, obj_in: OrderCreate) -> Order:
        # Validate user exists
        user = await self._user_repo.get_by_id(obj_in.user_id)
        if not user:
            raise NotFoundError(resource="User", id=obj_in.user_id)

        # Validate all items exist
        items = await self._item_repo.get_by_ids(obj_in.item_ids)
        if len(items) != len(obj_in.item_ids):
            raise ValidationError("Some items not found")

        return await super().create(obj_in)

4. Events/Hooks Pattern

class ItemService(BaseService[Item, ItemCreate, ItemUpdate]):
    async def create(self, obj_in: ItemCreate) -> Item:
        item = await super().create(obj_in)
        await self._on_created(item)
        return item

    async def _on_created(self, item: Item) -> None:
        """Hook called after item creation."""
        # Send notification, update cache, emit event, etc.
        logger.info(f"Item created: {item.id}")

5. Transaction Boundaries

By default, each repository method commits its transaction. For complex operations spanning multiple writes, consider transaction management:

# In repository, add a method that doesn't commit:
async def create_no_commit(self, obj_in: CreateSchemaType) -> ModelType:
    data = obj_in.model_dump()
    instance = self._model(**data)
    self._session.add(instance)
    await self._session.flush()  # Get ID without commit
    return instance

# In service:
async def create_order_with_items(self, order: OrderCreate) -> Order:
    async with self._session.begin():  # Transaction
        order = await self._order_repo.create_no_commit(order)
        for item in order.items:
            await self._order_item_repo.create_no_commit(item)
    # Commits on exit
    return order

Key Principles

1. Services contain business logic - validation, orchestration, rules
2. Services delegate data access - never write SQL in services
3. Services are stateless - no instance state between calls
4. Services raise domain exceptions - NotFoundError, ConflictError, etc.
5. Services are testable - mock the repository for unit tests

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