Support ActiveRecord transitions which don't include Statesman::Adapt…

…ers::ActiveRecordTransition

In #306, we made the "updated timestamp column" for a transition
configurable through a `class_attribute` on
`Statesman::Adapters::ActiveRecordTransition`, which is generally
`include`d on transition classes.

However, not all transition classes include this module, and
indeed we have [advised][1] in the README not to include it when
using a PostgreSQL JSON column type for metadata, as pointed out
by #309.

This leads to an exception like the following when trying to
perform a transition:

```
NoMethodError: undefined method `updated_timestamp_column' for #<Class:0x00007fdd2ade07b0>
```

This fixes the issue by defaulting to using `:updated_at` as
before if the transition class doesn't have an
`.updated_timestamp_column` method defined.

As a follow-up action, it'd be nice to get all transitions
including the module to have a more consistent and friendly base
for future improvements to Statesman, as discussed in the
[issue comments][2]. This, however, would be a breaking change
and should be introduced separately after more thought.

Fixes #309.

[1]: https://github.com/gocardless/statesman#using-postgresql-json-column
[2]: #309 (comment)

lib/statesman/adapters/active_record.rb

@@ -123,15 +123,28 @@ def transition_conflict_error?(e)
       end
 
       def with_updated_timestamp(params)
-        return params if @transition_class.updated_timestamp_column.nil?
+        # TODO: Once we've set expectations that transition classes should conform to
+        # the interface of Adapters::ActiveRecordTransition as a breaking change in the
+        # next major version, we can stop calling `#respond_to?` first and instead
+        # assume that there is a `.updated_timestamp_column` method we can call.
+        #
+        # At the moment, most transition classes will include the module, but not all,
+        # not least because it doesn't work with PostgreSQL JSON columns for metadata.
+        column = if @transition_class.respond_to?(:updated_timestamp_column)
+                   @transition_class.updated_timestamp_column
+                 else
+                   ActiveRecordTransition::DEFAULT_UPDATED_TIMESTAMP_COLUMN
+                 end
+
+        return params if column.nil?
 
         timestamp = if ::ActiveRecord::Base.default_timezone == :utc
                       Time.now.utc
                     else
                       Time.now
                     end
 
-        params.merge(@transition_class.updated_timestamp_column => timestamp)
+        params.merge(column => timestamp)
       end
     end
   end

spec/statesman/adapters/active_record_spec.rb

@@ -163,6 +163,19 @@
             to(change { previous_transition.reload.updated_at })
         end
 
+        context "for a transition class without an updated timestamp column attribute" do
+          let!(:adapter) do
+            described_class.new(MyActiveRecordModelTransitionWithoutInclude,
+                                model,
+                                observer)
+          end
+
+          it "defaults to touching the previous transition's updated_at timestamp" do
+            expect { Timecop.freeze(Time.now + 5.seconds) { create } }.
+              to(change { previous_transition.reload.updated_at })
+          end
+        end
+
         context "with a custom updated timestamp column set" do
           around do |example|
             MyActiveRecordModelTransition.updated_timestamp_column.tap do |original_value|

spec/support/active_record.rb

@@ -40,6 +40,13 @@ class MyActiveRecordModelTransition < ActiveRecord::Base
   serialize :metadata, JSON
 end
 
+class MyActiveRecordModelTransitionWithoutInclude < ActiveRecord::Base
+  self.table_name = "my_active_record_model_transitions"
+
+  belongs_to :my_active_record_model
+  serialize :metadata, JSON
+end
+
 class CreateMyActiveRecordModelMigration < MIGRATION_CLASS
   def change
     create_table :my_active_record_models do |t|