why PG::UniqueViolation: ERROR: duplicate key value violates unique constraint?

Question

I have a model Post and each time a post is created I want a new instance of Moderation to be created at the same time.

So in post.rb I use the

Answer 1

Looks like you've added a unique index to your database:

t.index ["reportable_type", "reportable_id"], name: "moderations_reportable", unique: true

With a unique index you will only be able to have one record with the same reportable_type and reportable_id. It's likely that you're trying to create a moderation for a reportable that already has a moderation.