Add method_call_assertions and use them instead of Mocha

Six Mocha calls prove quite resistant to Minitestification. For example,
if we replace

```
  ActiveRecord::Associations::HasManyAssociation
    .any_instance
    .expects(:reader)
    .never
```

with `assert_not_called`, Minitest wisely raises

```
NameError: undefined method `reader' for class `ActiveRecord::Associations::HasManyAssociation'
```

as `:reader` comes from a deeply embedded abstract class,
`ActiveRecord::Associations::CollectionAssociation`.

This patch tackles this difficulty by adding
`ActiveSupport::Testing::MethodCallAsserts#assert_called_on_instance_of`
which injects a stubbed method into `klass`, and verifies the number of
times it is called, similar to `assert_called`. It also adds  a convenience
method, `assert_not_called_on_instance_of`, mirroring
`assert_not_called`.

It uses the new method_call_assertions to replace the remaining Mocha
calls in `ActiveRecord` tests.

[utilum + bogdanvlviv + kspath]

1 files changed, 29 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/testing/method_call_assertions.rb b/activesupport/lib/active_support/testing/method_call_assertions.rb
index c6358002ea..fdc70e1cd3 100644
--- a/activesupport/lib/active_support/testing/method_call_assertions.rb
+++ b/activesupport/lib/active_support/testing/method_call_assertions.rb
@@ -35,6 +35,35 @@ module ActiveSupport
           assert_called(object, method_name, message, times: 0, &block)
         end
 
+        # TODO: No need to resort to #send once support for Ruby 2.4 is
+        # dropped.
+        def assert_called_on_instance_of(klass, method_name, message = nil, times: 1, returns: nil)
+          times_called = 0
+          klass.send(:define_method, "stubbed_#{method_name}") do |*|
+            times_called += 1
+
+            returns
+          end
+
+          klass.send(:alias_method, "original_#{method_name}", method_name)
+          klass.send(:alias_method, method_name, "stubbed_#{method_name}")
+
+          yield
+
+          error = "Expected #{method_name} to be called #{times} times, but was called #{times_called} times"
+          error = "#{message}.\n#{error}" if message
+
+          assert_equal times, times_called, error
+        ensure
+          klass.send(:alias_method, method_name, "original_#{method_name}")
+          klass.send(:undef_method, "original_#{method_name}")
+          klass.send(:undef_method, "stubbed_#{method_name}")
+        end
+
+        def assert_not_called_on_instance_of(klass, method_name, message = nil, &block)
+          assert_called_on_instance_of(klass, method_name, message, times: 0, &block)
+        end
+
         def stub_any_instance(klass, instance: klass.new)
           klass.stub(:new, instance) { yield instance }
         end