Add ability to specify how a class is converted to Arel predicate

This adds the ability for rails apps or gems to have granular control
over how a domain object is converted to sql. One simple use case would
be to add support for Regexp. Another simple case would be something
like the following:

    class DateRange < Struct.new(:start, :end)
      def include?(date)
        (start..end).cover?(date)
      end
    end

    class DateRangePredicate
      def call(attribute, range)
        attribute.in(range.start..range.end)
      end
    end

    ActiveRecord::PredicateBuilder.register_handler(DateRange,
      DateRangePredicate.new)

More complex cases might include taking a currency object and converting
it from EUR to USD before performing the query.

By moving the existing handlers to this format, we were also able to
nicely refactor a rather nasty method in PredicateBuilder.

1 files changed, 13 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb b/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb
new file mode 100644
index 0000000000..618fa3cdd9
--- /dev/null
+++ b/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb
@@ -0,0 +1,13 @@
+module ActiveRecord
+  class PredicateBuilder
+    class RelationHandler # :nodoc:
+      def call(attribute, value)
+        if value.select_values.empty?
+          value = value.select(value.klass.arel_table[value.klass.primary_key])
+        end
+
+        attribute.in(value.arel.ast)
+      end
+    end
+  end
+end