Dp Lazy Loading
DodaTech
1 min read
In this tutorial, you'll learn about How to Fix Lazy Loading Errors. We cover key concepts, practical examples, and best practices.
Fix lazy loading errors when eager load everything or lazy load with N+1 query problem.
Quick Fix
Wrong
class Author:
def __init__(self,id):
self.id=id; self.books=Book.find_by_author(id) # eager!
# Authors screen loads 100 authors -> triggers 100 book queries + 1 author query = 101 queries!
N+1 problem: 1 query for authors + N queries for each author's books.
Right
class Author:
def __init__(self,id,conn):
self.id=id; self.conn=conn; self._books=None
@property
def books(self):
if self._books is None:
cur=self.conn.execute('SELECT * FROM books WHERE author_id=?',(self.id,))
self._books=[{'id':r[0],'title':r[1]} for r in cur]
return self._books
# Or batch-load:
class AuthorRepository:
def get_with_books(self,ids):
# Single query for all books
cur=conn.execute('SELECT * FROM books WHERE author_id IN ({})'.format(','.join('?'*len(ids))),ids)
books={}; [books.setdefault(r[2],[]).append({'id':r[0],'title':r[1]}) for r in cur]
return books
Lazy: books loaded only when accessed. Batch: single query for all related data.
Prevention
Lazy loading defers initialization until needed. Batch loading solves N+1.
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