ActiveRecord query extension: where_exists(related,*where_params)

where_exists.rb

module ActiveRecord
  module FinderMethods
    
    # This simple class acts as a "stand-in" for your association, enabling us to reuse logic
    # in ::ActiveRecord::Associations::AssociationScope.
    class ExistingAssociation < Struct.new(:klass, :owner, :reflection)
      delegate :interpolate, to: :klass
      
      def initialize klass, related
        raise ArgumentError if Hash===related && related.length > 1
        related, subjoins = Array(related).flatten
        reflection = klass.reflect_on_association(related)
        super reflection.klass, klass.arel_table, reflection
      end
      
      def scope
        ::ActiveRecord::Associations::AssociationScope.new(self).scope
      end
    end
    
    # Build a WHERE clause that checks the existance of a related association or relation.
    # Any (optional) following arguments will be treated as separate invocations of where(*arg),
    # with the additional convenience that a Symbol on the right-hand side will be treated as column
    # from this model's table.
    def where_exists(related, *where_params)
      relation = 
        case related
        when ActiveRecord::Relation then related
        else ExistingAssociation.new(@klass, related).scope
        end
      relation = relation.send(:construct_relation_for_association_find, 
                               relation.send(:construct_join_dependency_for_association_find)).
                          except(:select, :order).
                          select("1 AS one")
      where("EXISTS (#{where_params.inject relation do |subq,where_param|
                         if not Hash===where_param then subq.where(where_param) else
                           subq.where(Hash[ where_param.map do |k,v|
                             [k, Symbol===v ? @klass.arel_table[v] : v]
                           end ])
                         end
                       end.to_sql})")
    end
  end
  
  module Querying
    delegate :where_exists, to: :scoped
  end
end