When dealing with legacy schemas or data models where the Active Record pattern is a bit of a stretch complex joins are often necessary. Many joins can be eliminated by using set operations on multiple queries.
Arel already supports set operations, and the Set module from the standard library already provide useful semantics.
Would this be something that could be of general use?
I’ve created a gem that patches these operations into ActiveRecord::Relation (https://github.com/delonnewman/activerecord-setops), and tested it with Rails 5. It’s very little code:
# Performs a set theoretic union works like
Array#+ but puts the load on the database
# and allows you to chain more relation operations.
alias | union
alias + union
def union_all(other) binary_operation(Arel::Nodes::UnionAll, other) end def intersect(other) binary_operation(Arel::Nodes::Intersect, other) end alias & intersect def except(other) binary_operation(Arel::Nodes::Except, other) end alias difference except alias - except private def binary_operation(op_class, other) @klass.unscoped.from(Arel::Nodes::TableAlias.new(op_class.new(self.arel.ast, other.arel.ast), @klass.arel_table.name)) end