MongoDB or PostgreSQL for a Q&A or social app?

Social and Q&A data is actually highly relational: users ↔ posts ↔ comments ↔ votes ↔ tags, plus follows, notifications and reputation. You'll constantly query *across* those relationships ("questions with tag X that the user hasn't voted on, sorted by score"). That's where relational databases shine.

PostgreSQL gives you joins, foreign keys (no orphaned comments), unique constraints (one vote per user), transactions across tables, and JSONB for the genuinely flexible parts.

MongoDB is strong when data is naturally document-shaped and read as a whole (product catalogs with varying attributes, event logs, CMS content), or when you need its horizontal-scaling model.

For a Q&A app, Postgres is the better default. It's exactly what Stack Overflow-style systems rely on (Stack Overflow itself runs on SQL Server).