Export postgres table based on rails relation

Question

I have a user table that I can easily export with postgres by doing

\copy users to 'tmp/users2015.csv' csv header

My users has_many questions, and I'm wondering if there's a way I can export just the users who have asked a question. The problem is here's no column in my users table that represents the number of questions a user has asked, so I don't think I can do this. Is there any possible way or a different approach? I really care about just knowing how many users have asked questions, so I could also export my questions table, and possibly filter out questions that have been asked by the same user?

Answer 1

You can execute the same select query that the has_many relationship uses and use that to write to a file.

Copy (Select * From users where <some condition>) To '/tmp/test.csv' With CSV

See Save PL/pgSQL output from PostgreSQL to a CSV file for more info.

Answer 2

Your should export in additional to the user table a table which has a belongs_to field, for example of question table, to 'tmp/question2015.csv' CSV.