Sqlalchemy with postgres. Try to get 'DISTINCT ON' instead of 'DISTINCT'

问题

I need to generate query like this:

SELECT **DISTINCT ON** (article.code) article.code, article.title

First I try to make it via ORM distinct method and send it a list with fields. But it wont work. Second, I try to make it via sqlalchemy.sql.select - and it also generate sql query like this:

SELECT DISTINCT article.code, article.title

I Need SELECT **DISTINCT ON** (article.code)...

I look at source code and found in sqlalchemy.dialects.postgresql.base.PGCompiler.get_select_precolumns code for generating constructions like: 'DISTINCT ON' But this method do not called. Instead of this called another method - sqlalchemy.sql.compiler.get_select_precolumns - it hasn't code for generating DISTINCT ON only for DISTINCT Maybe I should configure my session to called properly method?

回答1:

This bug report suggests that DISTINCT ON works correctly in SQLAlchemy 0.7+. I think an upgrade is in order, unless you've uncovered a bug in 0.7.

Workarounds . . .

• Volunteer to help get the 0.7 package ready for Ubuntu.
• Download and install from source.
• Rewrite queries to avoid DISTINCT ON. I'm not sure whether that's possible in the most general case.

来源：https://stackoverflow.com/questions/6175657/sqlalchemy-with-postgres-try-to-get-distinct-on-instead-of-distinct