Add an explicit test for hot compatibility

activerecord/test/cases/hot_compatibility_test.rb

+require 'cases/helper'
+
+class HotCompatibilityTest < ActiveRecord::TestCase
+  self.use_transactional_fixtures = false
+
+  setup do
+    @klass = Class.new(ActiveRecord::Base) do
+      connection.create_table :hot_compatibilities do |t|
+        t.string :foo
+        t.string :bar
+      end
+
+      def self.name; 'HotCompatibility'; end
+    end
+  end
+
+  teardown do
+    @klass.connection.drop_table :hot_compatibilities
+  end
+
+  test "insert after remove_column" do
+    # warm cache
+    @klass.create!
+
+    # we have 3 columns
+    assert_equal 3, @klass.columns.length
+
+    # remove one of them
+    @klass.connection.remove_column :hot_compatibilities, :bar
+
+    # we still have 3 columns in the cache
+    assert_equal 3, @klass.columns.length
+
+    # but we can successfully create a record so long as we don't
+    # reference the removed column
+    record = @klass.create! foo: 'foo'
+    record.reload
+    assert_equal 'foo', record.foo
+  end
+
+  test "update after remove_column" do
+    record = @klass.create! foo: 'foo'
+    assert_equal 3, @klass.columns.length
+    @klass.connection.remove_column :hot_compatibilities, :bar
+    assert_equal 3, @klass.columns.length
+
+    record.reload
+    assert_equal 'foo', record.foo
+    record.foo = 'bar'
+    record.save!
+    record.reload
+    assert_equal 'bar', record.foo
+  end
+end