Porting complicated has_many relationships to Rails >4.1 (without finder_sql)

Question

I am porting a Rails app to Rails 4.2. This Rails app contains some rather complex manual SQL code in associations - partly due to DB optimizations (e.g. subselects instead

Answer 1

Update

I learnt a while ago that this makes no sense. A has_many relationship must have injective connections at least in one direction, so an "OR" in a SQL clause makes no sense. How should a CREATE operation decide which condition to satisfy to create a new record? This relationship is read only by definition and so it is not a has_many relationship.

In this case, a simple class method (or scope) would be the right answer instead of has_many. To concatenate results from several queries use something like

def internal_messages
  InternalMessage.where( id: sent_message_ids + received_message_ids)
end

to keep the resulting object chainable (i.e. @user.internal_messages.by_date etc.)

Answer 2

Pass the proc that contains the SQL string as scope.

has_many :internal_messages, -> { proc { "SELECT DISTINCT(internal_messages.id), internal_messages.* FROM internal_messages " +
      ' LEFT JOIN internal_messages_recipients ON internal_messages.id=internal_messages_recipients.internal_message_id' +
      ' WHERE internal_messages.sender_id = #{id} OR internal_messages_recipients.recipient_id = #{id}' } }