Many-to-many relationships in Flask - Performance & Optimization

Performance: Many-to-many relationships

MEDIUM IMPACT

This affects how quickly the page loads and updates when displaying or modifying related data across multiple tables.

Displaying related items in a many-to-many relationship

Flask

from sqlalchemy.orm import joinedload
posts = Post.query.options(joinedload(Post.tags)).all()
for post in posts:
    tags = post.tags  # loaded in one query
    for tag in tags:
        print(tag.name)

Loads all posts and their tags in a single query, avoiding repeated database hits.

📈 Performance GainReduces queries from N+1 to 1, improving LCP and reducing server response time.

Displaying related items in a many-to-many relationship

Flask

posts = Post.query.all()
for post in posts:
    tags = post.tags  # lazy loading inside loop
    for tag in tags:
        print(tag.name)

Triggers a database query for each post to load tags, causing N+1 query problem.

📉 Performance CostBlocks rendering due to multiple sequential database queries, increasing LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Lazy loading in loop (N+1 queries)	Minimal DOM nodes	1 reflow after data load	Low paint cost	[X] Bad
Eager loading with joinedload	Minimal DOM nodes	1 reflow after data load	Low paint cost	[OK] Good

Rendering Pipeline

Many-to-many relationships require fetching related data from multiple tables, which affects the data retrieval stage before rendering. Inefficient queries delay data availability, blocking the rendering pipeline.

→Data Fetching

→Rendering

→Layout

⚠️ BottleneckData Fetching due to multiple database queries

Core Web Vital Affected

LCP

This affects how quickly the page loads and updates when displaying or modifying related data across multiple tables.

Optimization Tips

1Avoid lazy loading related data inside loops to prevent multiple database queries.

2Use eager loading (e.g., joinedload) to fetch many-to-many related data in one query.

3Reducing database queries improves Largest Contentful Paint and overall page speed.

Performance Quiz - 3 Questions

What is the main performance problem with lazy loading many-to-many relationships inside a loop?

A It increases the number of DOM nodes unnecessarily.

B It causes multiple database queries, slowing down page load.

C It blocks CSS from loading.

D It causes excessive JavaScript execution.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and count the number of API/database calls made to fetch related data.

What to look for: Multiple repeated calls for related data indicate N+1 query problem; a single combined call indicates good performance.