Promote Order#all_adjustments to association.

Fixes spree#6391

core/app/models/spree/adjustment.rb

@@ -24,7 +24,7 @@ module Spree
   class Adjustment < Spree::Base
     belongs_to :adjustable, polymorphic: true, touch: true
     belongs_to :source, polymorphic: true
-    belongs_to :order, class_name: "Spree::Order"
+    belongs_to :order, class_name: 'Spree::Order', inverse_of: :all_adjustments
 
     validates :adjustable, presence: true
     validates :order, presence: true

core/app/models/spree/order.rb

@@ -70,6 +70,11 @@ def generate_number(options = {})
     has_many :products, through: :variants
     has_many :variants, through: :line_items
     has_many :refunds, through: :payments
+    has_many :all_adjustments,
+             class_name: 'Spree::Adjustment',
+             foreign_key: :order_id,
+             dependent: :destroy,
+             inverse_of: :order
 
     has_and_belongs_to_many :promotions, join_table: 'spree_orders_promotions'
 
@@ -132,11 +137,6 @@ def self.register_line_item_comparison_hook(hook)
       self.line_item_comparison_hooks.add(hook)
     end
 
-    def all_adjustments
-      Adjustment.where("order_id = :order_id OR (adjustable_id = :order_id AND adjustable_type = 'Spree::Order')",
-                       order_id: self.id)
-    end
-
     # For compatiblity with Calculator::PriceSack
     def amount
       line_items.inject(0.0) { |sum, li| sum + li.amount }

core/db/migrate/20150515211137_fix_adjustment_order_id.rb

@@ -0,0 +1,70 @@
+class FixAdjustmentOrderId < ActiveRecord::Migration
+  def change
+    say 'Populate order_id from adjustable_id where appropriate'
+    execute(<<-SQL.squish)
+      UPDATE
+        spree_adjustments
+      SET
+        order_id = adjustable_id
+      WHERE
+        adjustable_type = 'Spree::Order'
+      ;
+    SQL
+
+    # Submitter of change does not care about MySQL, as it is not officially supported.
+    # Still spree officials decided to provide a working code path for MySQL users, hence
+    # submitter made a AR code path he could validate on PostgreSQL.
+    #
+    # Whoever runs a big enough MySQL installation where the AR solution hurts:
+    # Will have to write a better MySQL specific equivalent.
+    if Spree::Order.connection.adapter_name.eql?('MySQL')
+      Spree::Adjustment.where(adjustable_type: 'Spree::LineItem').find_each do |adjustment|
+        adjustment.update_columns(order_id: Spree::LineItem.find(adjustment.adjustable_id).order_id)
+      end
+    else
+      execute(<<-SQL.squish)
+        UPDATE
+          spree_adjustments
+        SET
+          order_id =
+            (SELECT order_id FROM spree_line_items WHERE spree_line_items.id = spree_adjustments.adjustable_id)
+        WHERE
+          adjustable_type = 'Spree::LineItem'
+      SQL
+    end
+
+    say 'Fix schema for spree_adjustments order_id column'
+    change_table :spree_adjustments do |t|
+      t.change :order_id, :integer, null: false
+    end
+
+    # Improved schema for postgresql, uncomment if you like it:
+    #
+    # # Negated Logical implication.
+    # #
+    # # When adjustable_type is 'Spree::Order' (p) the adjustable_id must be order_id (q).
+    # #
+    # # When adjustable_type is NOT 'Spree::Order' the adjustable id allowed to be any value (including of order_id in
+    # # case foreign keys match). XOR does not work here.
+    # #
+    # # Postgresql does not have an operator for logical implication. So we need to build the following truth table
+    # # via AND with OR:
+    # #
+    # #  p q | CHECK = !(p -> q)
+    # #  -----------
+    # #  t t | t
+    # #  t f | f
+    # #  f t | t
+    # #  f f | t
+    # #
+    # # According to de-morgans law the logical implication q -> p is equivalent to !p || q
+    # #
+    # execute(<<-SQL.squish)
+    #   ALTER TABLE ONLY spree_adjustments
+    #    ADD CONSTRAINT fk_spree_adjustments FOREIGN KEY (order_id)
+    #      REFERENCES spree_orders(id) ON UPDATE RESTRICT ON DELETE RESTRICT,
+    #    ADD CONSTRAINT check_spree_adjustments_order_id CHECK
+    #      (adjustable_type <> 'Spree::Order' OR order_id = adjustable_id);
+    # SQL
+  end
+end

core/spec/models/spree/order/adjustments_spec.rb

@@ -1,16 +1,6 @@
 require 'spec_helper'
 
-describe Spree::Order, :type => :model do
-  context "#all_adjustments" do
-    # Regression test for #4537
-    it "does not show adjustments from other, non-order adjustables" do
-      order = Spree::Order.new(:id => 1)
-      where_sql = order.all_adjustments.where_values.to_s
-      expect(where_sql).to include("(adjustable_id = 1 AND adjustable_type = 'Spree::Order')")
-    end
-  end
-
-  # Regression test for #2191
+describe Spree::Order do
   context "when an order has an adjustment that zeroes the total, but another adjustment for shipping that raises it above zero" do
     let!(:persisted_order) { create(:order) }
     let!(:line_item) { create(:line_item) }