make filter column objects flyweight

lib/active_interaction/base.rb

@@ -166,7 +166,7 @@ def initialize(inputs = {})
 
     def column_for_attribute(name)
       filter = self.class.filters[name]
-      FilterColumn.new(filter.database_column_type) if filter
+      FilterColumn.intern(filter.database_column_type) if filter
     end
 
     # @!method compose(other, inputs = {})

lib/active_interaction/filter_column.rb

@@ -5,6 +5,20 @@ module ActiveInteraction
   class FilterColumn
     attr_reader :limit, :type
 
+    class << self
+      def intern(type)
+        @columns ||= {}
+
+        if @columns[type]
+          @columns.fetch(type)
+        else
+          @columns[type] = new(type)
+        end
+      end
+
+      private :new
+    end
+
     def initialize(type)
       @type = type
     end

spec/active_interaction/filter_column_spec.rb

@@ -4,7 +4,23 @@
 
 describe ActiveInteraction::FilterColumn do
   let(:type) { :float }
-  subject(:column) { described_class.new(type) }
+  subject(:column) { described_class.intern(type) }
+
+  describe '.intern(type)' do
+    it 'returns the same object for each type' do
+      expect(described_class.intern(type)).to equal column
+    end
+
+    it 'returns different objects for different types' do
+      expect(described_class.intern(:integer)).to_not equal column
+    end
+  end
+
+  describe '.new(type)' do
+    it 'is private' do
+      expect { described_class.new(type) }.to raise_error NoMethodError
+    end
+  end
 
   describe '#limit' do
     it 'returns nil' do