python 51 lines · 9 steps

Cursor pagination in a FastAPI endpoint

An articles endpoint pages through rows with opaque, base64-encoded cursors instead of numeric offsets.

Explained by highlit
FastAPI Intermediate pagination cursor async base64 query-parameters
1import base64
2import json
3from typing import Annotated, Optional
4 
5from fastapi import APIRouter, Depends, HTTPException, Query
6from pydantic import BaseModel
7from sqlalchemy import select
8from sqlalchemy.ext.asyncio import AsyncSession
9 
10from .database import get_session
11from .models import Article
12from .schemas import ArticleOut
13 
14router = APIRouter(prefix="/articles", tags=["articles"])
15 
16 
17def encode_cursor(article_id: int) -> str:
18 raw = json.dumps({"id": article_id}).encode()
19 return base64.urlsafe_b64encode(raw).decode()
20 
21 
22def decode_cursor(cursor: str) -> int:
23 try:
24 payload = json.loads(base64.urlsafe_b64decode(cursor.encode()))
25 return int(payload["id"])
26 except (ValueError, KeyError, json.JSONDecodeError):
27 raise HTTPException(status_code=400, detail="Invalid pagination cursor")
28 
29 
30class ArticlePage(BaseModel):
31 items: list[ArticleOut]
32 next_cursor: Optional[str] = None
33 
34 
35@router.get("", response_model=ArticlePage)
36async def list_articles(
37 session: Annotated[AsyncSession, Depends(get_session)],
38 cursor: Optional[str] = None,
39 limit: int = Query(20, ge=1, le=100),
40):
41 stmt = select(Article).order_by(Article.id.asc()).limit(limit + 1)
42 if cursor is not None:
43 stmt = stmt.where(Article.id > decode_cursor(cursor))
44 
45 rows = (await session.scalars(stmt)).all()
46 
47 has_more = len(rows) > limit
48 page = rows[:limit]
49 next_cursor = encode_cursor(page[-1].id) if has_more else None
50 
51 return ArticlePage(items=page, next_cursor=next_cursor)
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Cursor pagination keeps queries efficient at scale because it filters by a keyed boundary instead of skipping rows with OFFSET.
  2. 2Fetching limit+1 rows is a cheap trick to detect whether another page exists without a separate count query.
  3. 3Encoding the cursor keeps its shape opaque to clients, so you can change what it contains without breaking their code.

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