How to make Active Record join return unique objects?

问题

I have a simple query need: Find a list of users who made an order since Jan 1, 2013.

In SQL, it's a very simple query.

But I'm using Rails and Active Record.

So I wrote: User.joins(:orders).where("orders.created_at >= '2013-01-01 00:00:00'")

In our database, we have 100 orders made since 01/01/2013 by 75 users. (Some users made more than one order apparently.)

However, the expression above returns 100 users. (There must be duplicates.)

I tried User.joins(:orders).where("orders.created_at >= '2013-01-01 00:00:00'").uniq

That doesn't work either.

How can I get the 75 users who've made an order since 01/01/2013?

回答1:

@dbjohn has the right idea, but I assume you want to avoid creating extra objects. Here's a slight variant on his solution, letting the database do the uniq-ing for you:

date = "2013-01-01 00:00:00"
User.joins(:orders).where("orders.created_at >= ?", date).distinct

Note that you can rearrange the order of methods to fit whatever you think is most semantic, and ActiveRecord will write the same SQL for you.

回答2:

User.joins(:orders).
    where("orders.created_at >= '2013-01-01 00:00:00'").
    group('users.id')

group method will chain to the query and give you a list of unique records.

回答3:

Rails has added uniq since version 3.2.1 so now you can use uniq

http://apidock.com/rails/ActiveRecord/QueryMethods/uniq

来源：https://stackoverflow.com/questions/15230568/how-to-make-active-record-join-return-unique-objects