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, 14 insertions, 0 deletions

diff --git a/activerecord/test/cases/relation/predicate_builder_test.rb b/activerecord/test/cases/relation/predicate_builder_test.rb
new file mode 100644
index 0000000000..1c87f5c12e
--- /dev/null
+++ b/activerecord/test/cases/relation/predicate_builder_test.rb
@@ -0,0 +1,14 @@
+require "cases/helper"
+require 'models/topic'
+
+module ActiveRecord
+  class PredicateBuilderTest < ActiveRecord::TestCase
+    def test_registering_new_handlers
+      PredicateBuilder.register_handler(Regexp, proc do |column, value|
+        Arel::Nodes::InfixOperation.new('~', column, value.source)
+      end)
+
+      assert_match %r{["`]topics["`].["`]title["`] ~ 'rails'}, Topic.where(title: /rails/).to_sql
+    end
+  end
+end