Implementing Offset-Based Pagination with Strawberry GraphQL and Relay
Disclaimer: This article is written by Claude Opus completely. Refer to this article https://medium.com/@ryan-zheng/task-decomposition-and-execute-prompt-engineering-1296dcbc6114 to get the prompt. Of course, along the way, i have to ask Opus to add the missing code.
Introduction:
In this article, we will explore how to implement offset-based pagination in a GraphQL API using Strawberry GraphQL and Relay. Pagination is a crucial aspect of API design, especially when dealing with large datasets. Offset-based pagination allows clients to retrieve a subset of results by specifying an offset and limit, providing a simple and intuitive way to navigate through the data. While Relay is typically associated with cursor-based pagination, we can leverage its functionality to achieve offset-based pagination by utilizing the LazyQuery class.
Understanding Offset-Based Pagination:
Offset-based pagination works by specifying two parameters: offset and limit. The offset indicates the number of records to skip from the beginning of the result set, while the limit determines the maximum number of records to return. By adjusting these parameters, clients can retrieve different subsets of the data, allowing them to navigate through the results page by page.
Strawberry GraphQL:
Strawberry is a Python library for building GraphQL APIs. It provides a simple and intuitive way to define GraphQL schemas using Python classes and functions. With Strawberry, you can define your GraphQL types, queries, mutations, and subscriptions using familiar Python syntax. It leverages Python’s type hints for type safety and provides decorators to define resolvers for your GraphQL fields.
Relay-style Pagination in Strawberry GraphQL:
Strawberry GraphQL provides support for Relay-style pagination, which is a specification for implementing efficient pagination in GraphQL APIs. Relay-style pagination defines a standard way to structure connections and edges in the GraphQL schema, allowing clients to query paginated results using a connection-based approach.
In the provided code, the relay module from Strawberry GraphQL is used to define the connection types and implement pagination. The @relay.connection decorator is used to specify the connection type for the paginated query, and the ListConnection class is used to define the structure of the paginated results.
SQLAlchemy:
SQLAlchemy is a popular Python library for working with relational databases. It provides a set of tools for defining database models, querying data, and performing database operations. With SQLAlchemy, you can define your database models using Python classes and use an object-relational mapping (ORM) to interact with the database. It offers a powerful query API for expressing complex database queries using Python code.
Implementing Offset-Based Pagination with Relay:
To implement offset-based pagination in a GraphQL API using Strawberry GraphQL and Relay, follow these steps:
1. Define your database models using SQLAlchemy. Create a `BookModel` class that represents the structure of the book table in the database.
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class BookModel(Base):
__tablename__ = "books"
id = Column(Integer, primary_key=True)
title = Column(String)
author = Column(String)2. Create a base repository class (`BaseRepository`) that provides basic database operations and a `get_all_lazy` method that returns a `LazyQuery` object.
from typing import Generic, TypeVar
from sqlalchemy.orm import Query, Session
ModelType = TypeVar("ModelType")
class LazyQuery(Generic[ModelType]):
def __init__(self, query: Query):
self.query: Query = query
def __getitem__(self, key: slice) -> Query:
if isinstance(key, slice):
return self.query.slice(key.start, key.stop)
raise TypeError("LazyQuery only supports slicing.")
class BaseRepository(Generic[ModelType]):
model = None
@property
def db(self) -> Session:
return request_scoped_db_session.get()
def lazy_query(self, *criterion) -> LazyQuery[ModelType]:
query: Query = self.db.query(self.model)
if criterion:
query = query.filter(*criterion)
return LazyQuery(query)
def get_all_lazy(self) -> LazyQuery[ModelType]:
return self.lazy_query()3. Define a converter function (`model_to_book_graphql`) that converts a `BookModel` instance to a `book` GraphQL type.
from src.api.grapqhl.types.book_types import book
def model_to_book_graphql(book_model: BookModel) -> book:
return book(
id=strawberry.ID(str(book_model.id)),
title=book_model.title,
author=book_model.author,
)4. Define your GraphQL schema using Strawberry GraphQL. Create types for your data objects (e.g., `book` and `bookInput`) and a connection type (`bookListConnection`) that represents the paginated results.
import strawberry
from strawberry import relay
@strawberry.type
class book:
id: strawberry.ID
title: str
author: str
@strawberry.input
class bookInput:
title: str
author: str
@strawberry.type
class bookListConnection(relay.ListConnection[book]):
@classmethod
def resolve_node(cls, node: BookModel, *, info: Info, **kwargs) -> book:
return model_to_book_graphql(node)5. Implement a resolver function for the paginated query (e.g., `get_all_books`). This function should use the `LazyQuery` object to retrieve the data from the database and return it as a Relay connection.
from typing import Iterable
from strawberry.types import Info
from src.api.grapqhl.converters.book_converters import model_to_book_graphql
from src.db.repositories.book_repository import bookRepository
book_repository = bookRepository()
@strawberry.type
class Query:
@relay.connection(bookListConnection)
def get_all_books(self, info) -> Iterable[book]:
books = book_repository.get_all_lazy()
return books6. Use the `@relay.connection` decorator to specify the connection type for the paginated query. This decorator handles the pagination arguments and returns the data in the format expected by Relay-style pagination.
7. In the resolver function, call the `get_all_lazy` method from the base repository to obtain a `LazyQuery` object representing the query to fetch all data from the database.
8. The `ListConnection` class from Strawberry GraphQL takes care of slicing the `LazyQuery` object based on the pagination arguments provided by the client.
9. The sliced `LazyQuery` is executed, and the corresponding subset of data is retrieved from the database using SQLAlchemy.
10. The retrieved `BookModel` instances are converted to the appropriate GraphQL types using the `model_to_book_graphql` function in the `resolve_node` method of the connection class.
11. The paginated results are returned to the client, along with the necessary pagination information (e.g., `edges`, `pageInfo`) as defined by Relay-style pagination.
Conclusion:
Implementing offset-based pagination in a GraphQL API using Strawberry GraphQL and Relay provides a powerful and efficient way to handle large datasets. By leveraging the strengths of each technology and utilizing the LazyQuery class, we can achieve offset-based pagination while still benefiting from the structure and conventions provided by Relay.
Strawberry GraphQL simplifies the process of defining the GraphQL schema and resolvers while providing support for Relay-style pagination. SQLAlchemy offers a robust ORM for interacting with the database and performing efficient queries. The LazyQuery class enables slicing the query results, allowing for offset-based pagination behind the scenes.
By following the steps outlined in this article and utilizing the provided code examples, you can successfully implement offset-based pagination in your GraphQL API using Strawberry GraphQL and Relay, enhancing its usability and performance. The complete code implementation demonstrates how all the components work together to achieve efficient pagination of book results.
Feel free to copy and experiment with the code provided in this article to gain a hands-on understanding of offset-based pagination in a GraphQL API using Strawberry GraphQL and Relay.