Add ApplicationRecord.where_exists

This is a simple method for generating a WHERE EXISTS clause, without
having to write it out manually. This pattern is used in several places,
so this method allows us to cut down the amount of repetition in the
future.

app/models/application_record.rb

@@ -61,4 +61,8 @@ class ApplicationRecord < ActiveRecord::Base
   def self.underscore
     Gitlab::SafeRequestStore.fetch("model:#{self}:underscore") { self.to_s.underscore }
   end
+
+  def self.where_exists(query)
+    where('EXISTS (?)', query.select(1))
+  end
 end

spec/models/application_record_spec.rb

@@ -90,4 +90,12 @@ RSpec.describe ApplicationRecord do
       expect(User.at_most(2).count).to eq(2)
     end
   end
+
+  describe '.where_exists' do
+    it 'produces a WHERE EXISTS query' do
+      user = create(:user)
+
+      expect(User.where_exists(User.limit(1))).to eq([user])
+    end
+  end
 end