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.

2 files changed, 42 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation/predicate_builder/array_handler.rb b/activerecord/lib/active_record/relation/predicate_builder/array_handler.rb
new file mode 100644
index 0000000000..2f6c34ac08
--- /dev/null
+++ b/activerecord/lib/active_record/relation/predicate_builder/array_handler.rb
@@ -0,0 +1,29 @@
+module ActiveRecord
+  class PredicateBuilder
+    class ArrayHandler # :nodoc:
+      def call(attribute, value)
+        values = value.map { |x| x.is_a?(Base) ? x.id : x }
+        ranges, values = values.partition { |v| v.is_a?(Range) }
+
+        values_predicate = if values.include?(nil)
+          values = values.compact
+
+          case values.length
+          when 0
+            attribute.eq(nil)
+          when 1
+            attribute.eq(values.first).or(attribute.eq(nil))
+          else
+            attribute.in(values).or(attribute.eq(nil))
+          end
+        else
+          attribute.in(values)
+        end
+
+        array_predicates = ranges.map { |range| attribute.in(range) }
+        array_predicates << values_predicate
+        array_predicates.inject { |composite, predicate| composite.or(predicate) }
+      end
+    end
+  end
+end
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