rails 4 在 postgres 的多列上选择不同的地方和限制

Question

How do you select multiple fields with distinct values, and other non-distinct fields with them, all in one call with where and limit? I tried .pluck (which supports multiple fields in rails 4), .uniq (which didn't work in my case).

Answer 1

This is what worked for me, when used in the controller action

@models = Model.select('DISTINCT ON (field1,field2,field3) *')
     .where(id: params[:id])
     .limit(100)

Answer 2

这里有一个不那么冗长、更以 ActiveRecord 为中心的方法，它不仅适用于 Postgres，也适用于 MySQL。

Model.select('field1,field2').distinct.where(field3: 'value').limit(10)