Rails Postgres GROUP所有列

Question

You must GROUP BY all columns you are selecting by in Postgres or it will get mad at you. Is there a simple/clean way to do this other than just listing all the columns?

The best I've come up with is the following (intent is commented):

# The purpose of this query is to return a list of `User`s
# ordered by the number of `Account`s they have in descending order.

User.joins(:accounts)
    .group(User.full_column_names)
    .order('COUNT("accounts".*) DESC')

From user.rb :

def self.full_column_names
  column_names.collect {|name| "\"#{table_name}\".\"#{name}\"" }
end

Answer 1

One way to find the user with the most accounts:

user_id, number_of_accounts = Account.group(:user_id).count.max_by{ |key, value| value}
user_with_most_accounts = User.find(user_id)

Make it one query with:

user_with_most_accounts = User.find(Account.group(:user_id).count.max_by{ |key, value| value}[0])

Note : This finds only one user even if multiple users have the same number of accounts as the most count.

Answer 2

Enough time has passed where I have to say that unfortunately there is not a better solution for the specific question I was asking. Maybe Rails 5 will have something and I'll revisit this later. My basic solution in the question works for my usage and could be modified or used in another higher level method without much work to perform the grouping.

This may also be solved in a update to the Postgres adapter.