PostgreSQL vs MongoDB is the wrong first question

PostgreSQL and MongoDB are both mature databases. The useful question is not which one is better. It is which one fits the shape of your data, the reads you need and the guarantees you cannot give up.

Questions that actually decide it

• Are the relationships genuinely relational, with joins and multi-row transactions? PostgreSQL is usually the calmer default.
• Is the data a document read and written as a whole, with a bounded shape? MongoDB can map naturally.
• Do you need ad-hoc reporting and constraints across entities? Start relational and measure before denormalising.
• Is eventual consistency acceptable? If not, be explicit about the transaction boundary in either system.

-- PostgreSQL makes relationships and constraints explicit
select c.title, count(l.id) as lessons
from courses c
left join lessons l on l.course_slug = c.slug
group by c.title
having count(l.id) > 3;

A document database does not remove the need to model. It moves the shape decision into how you embed and reference. A relational database does not force a bad schema; it makes the constraints visible earlier.