Sort by default_sort on association class.

lib/administrate/order.rb

@@ -8,6 +8,8 @@ def initialize(attribute = nil, direction = nil)
     def apply(relation)
       if relation.columns_hash.keys.include?(attribute.to_s)
         relation.order(attribute => direction)
+      elsif association?(relation)
+        AssociationOrder.new(relation, attribute, direction).relation_with_order
       else
         relation
       end
@@ -30,6 +32,12 @@ def order_params_for(attr)
 
     attr_reader :attribute
 
+    def association?(relation)
+      if relation.respond_to? :reflect_on_association
+        relation.reflect_on_association(attribute)
+      end
+    end
+
     def reversed_direction_param_for(attr)
       if ordered_by?(attr)
         opposite_direction
@@ -42,4 +50,40 @@ def opposite_direction
       direction.to_sym == :asc ? :desc : :asc
     end
   end
+
+  class AssociationOrder
+    def initialize(relation, attribute, direction)
+      @relation = relation
+      @attribute = attribute
+      @direction = direction
+    end
+
+    def relation_with_order
+      association ? relation.includes(association.name).order("#{association.plural_name}.#{order_name} #{direction_for_sql}") : relation
+    end
+
+    def order_name
+      if association.klass.respond_to? :default_sort
+      association.klass.default_sort
+      else
+        :name
+      end
+    end
+
+    def direction_for_sql
+      if direction.to_sym == :asc
+        "ASC"
+      else
+        "DESC"
+      end
+    end
+
+    private
+
+    def association
+      relation.reflect_on_association(attribute)
+    end
+
+    attr_reader :relation, :attribute, :direction
+  end
 end

spec/lib/administrate/order_spec.rb

@@ -1,4 +1,5 @@
 require "administrate/order"
+require "rails_helper"
 
 describe Administrate::Order do
   describe "#apply" do
@@ -51,6 +52,33 @@
         expect(ordered).to eq(relation)
       end
     end
+
+    context 'when `order` uses an association with no default_sort method' do
+      it 'orders by `name` on the associated record by default' do
+        order = Administrate::Order.new(:customer, :desc)
+        relation = Order.all
+        allow(relation).to receive(:order).and_return(relation)
+
+        ordered = order.apply(relation)
+
+        expect(relation).to have_received(:order).with('customers.name DESC')
+        expect(ordered).to eq(relation)
+      end
+    end
+
+    context 'when `order` uses an association with a default_sort method' do
+      it 'orders by the attribute given in the default_sort method' do
+        order = Administrate::Order.new(:customer, :asc)
+        relation = Order.all
+        allow(relation).to receive(:order).and_return(relation)
+        allow(Customer).to receive(:default_sort).and_return(:updated_at)
+
+        ordered = order.apply(relation)
+
+        expect(relation).to have_received(:order).with('customers.updated_at ASC')
+        expect(ordered).to eq(relation)
+      end
+    end
   end
 
   describe "#ordered_by?" do