Fix MySQL most_recent column

lib/generators/statesman/generator_helpers.rb

@@ -36,5 +36,9 @@ def mysql?
       ActiveRecord::Base.configurations[Rails.env].
         try(:[], "adapter").try(:match, /mysql/)
     end
+
+    def database_supports_partial_indexes?
+      Statesman::Adapters::ActiveRecord.database_supports_partial_indexes?
+    end
   end
 end

lib/generators/statesman/templates/add_constraints_to_most_recent_migration.rb.erb

@@ -2,12 +2,18 @@ class AddConstraintsToMostRecentFor<%= migration_class_name %> < ActiveRecord::M
   disable_ddl_transaction!
 
   def up
+    <%- if database_supports_partial_indexes? -%>
     add_index :<%= table_name %>, [:<%= parent_id %>, :most_recent], unique: true, where: "most_recent", name: "index_<%= table_name %>_parent_most_recent", algorithm: :concurrently
     change_column_null :<%= table_name %>, :most_recent, false
+    <%- else -%>
+    add_index :<%= table_name %>, [:<%= parent_id %>, :most_recent], unique: true, name: "index_<%= table_name %>_parent_most_recent", algorithm: :concurrently
+    <%- end -%>
   end
 
   def down
     remove_index :<%= table_name %>, name: "index_<%= table_name %>_parent_most_recent"
+    <%- if database_supports_partial_indexes? -%>
     change_column_null :<%= table_name %>, :most_recent, true
+    <%- end -%>
   end
 end

lib/generators/statesman/templates/create_migration.rb.erb

@@ -5,11 +5,18 @@ class Create<%= migration_class_name %> < ActiveRecord::Migration
       t.text :metadata<%= ", default: \"{}\"" unless mysql? %>
       t.integer :sort_key, null: false
       t.integer :<%= parent_id %>, null: false
-      t.boolean :most_recent, null: false
+      t.boolean :most_recent<%= ", null: false" if database_supports_partial_indexes? %>
       t.timestamps null: false
     end
 
-    add_index :<%= table_name %>, [:<%= parent_id %>, :sort_key], unique: true, name: "<%= index_name :parent_sort %>"
-    add_index :<%= table_name %>, [:<%= parent_id %>, :most_recent], unique: true, where: "most_recent", name: "<%= index_name :parent_most_recent %>"
+    add_index(:<%= table_name %>,
+              [:<%= parent_id %>, :sort_key],
+              unique: true,
+              name: "<%= index_name :parent_sort %>")
+    add_index(:<%= table_name %>,
+              [:<%= parent_id %>, :most_recent],
+              unique: true,
+              <%= "where: 'most_recent'," if database_supports_partial_indexes? %>
+              name: "<%= index_name :parent_most_recent %>")
   end
 end

lib/statesman/adapters/active_record.rb

@@ -8,6 +8,15 @@ class ActiveRecord
 
       JSON_COLUMN_TYPES = %w(json jsonb).freeze
 
+      def self.database_supports_partial_indexes?
+        # Rails 3 doesn't implement `supports_partial_index?`
+        if ::ActiveRecord::Base.connection.respond_to?(:supports_partial_index?)
+          ::ActiveRecord::Base.connection.supports_partial_index?
+        else
+          ::ActiveRecord::Base.connection.adapter_name == 'PostgreSQL'
+        end
+      end
+
       def initialize(transition_class, parent_model, observer, options = {})
         serialized = serialized?(transition_class)
         column_type = transition_class.columns_hash['metadata'].sql_type
@@ -78,7 +87,18 @@ def transitions_for_parent
 
       def unset_old_most_recent
         return unless most_recent_column?
-        transitions_for_parent.update_all(most_recent: false)
+        # Check whether the `most_recent` column allows null values. If it
+        # doesn't, set old records to `false`, otherwise, set them to `NULL`.
+        #
+        # Some conditioning here is required to support databases that don't
+        # support partial indexes. By doing the conditioning on the column,
+        # rather than Rails' opinion of whether the database supports partial
+        # indexes, we're robust to DBs later adding support for partial indexes.
+        if transition_class.columns_hash['most_recent'].null == false
+          transitions_for_parent.update_all(most_recent: false)
+        else
+          transitions_for_parent.update_all(most_recent: nil)
+        end
       end
 
       def most_recent_column?

lib/tasks/statesman.rake

@@ -15,9 +15,14 @@ namespace :statesman do
 
     parent_class.find_in_batches(batch_size: batch_size) do |models|
       ActiveRecord::Base.transaction do
-        # Set all transitions' most_recent to FALSE
-        transition_class.where(parent_fk => models.map(&:id)).
-          update_all(most_recent: false)
+        if transition_class.columns_hash['most_recent'].null == false
+          # Set all transitions' most_recent to FALSE
+          transition_class.where(parent_fk => models.map(&:id)).
+            update_all(most_recent: false)
+        else
+          transition_class.where(parent_fk => models.map(&:id)).
+            update_all(most_recent: nil)
+        end
 
         # Set current transition's most_recent to TRUE
         ActiveRecord::Base.connection.execute %{

spec/fixtures/add_constraints_to_most_recent_for_bacon_transitions_without_partial_index.rb

@@ -0,0 +1,11 @@
+class AddConstraintsToMostRecentForBaconTransitions < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def up
+    add_index :bacon_transitions, [:bacon_id, :most_recent], unique: true, name: "index_bacon_transitions_parent_most_recent", algorithm: :concurrently
+  end
+
+  def down
+    remove_index :bacon_transitions, name: "index_bacon_transitions_parent_most_recent"
+  end
+end

spec/generators/statesman/add_constraints_to_most_recent_generator_spec.rb

@@ -22,8 +22,13 @@
     end
 
     let(:fixture_file) do
-      File.read("spec/fixtures/add_constraints_to_most_recent_for_"\
-                "bacon_transitions.rb")
+      if Statesman::Adapters::ActiveRecord.database_supports_partial_indexes?
+        File.read("spec/fixtures/add_constraints_to_most_recent_for_"\
+                  "bacon_transitions_with_partial_index.rb")
+      else
+        File.read("spec/fixtures/add_constraints_to_most_recent_for_"\
+                  "bacon_transitions_without_partial_index.rb")
+      end
     end
 
     before do

spec/statesman/adapters/active_record_spec.rb

@@ -145,7 +145,7 @@
         it "updates the previous transition's most_recent flag" do
           expect { create }.
             to change { previous_transition.reload.most_recent }.
-            from(true).to(false)
+            from(true).to be_falsey
         end
 
         context "and the parent model is updated in a callback" do
@@ -162,6 +162,18 @@
           end
         end
       end
+
+      context "with two previous transitions" do
+        let!(:previous_transition) { adapter.create(from, to) }
+        let!(:another_previous_transition) { adapter.create(from, to) }
+        its(:most_recent) { is_expected.to eq(true) }
+
+        it "updates the previous transition's most_recent flag" do
+          expect { create }.
+            to change { another_previous_transition.reload.most_recent }.
+            from(true).to be_falsey
+        end
+      end
     end
 
     context "when the transition_class doesn't have a most_recent column" do

spec/support/active_record.rb

@@ -50,7 +50,6 @@ def change
       t.string  :to_state
       t.integer :my_active_record_model_id
       t.integer :sort_key
-      t.boolean :most_recent, default: true, null: false
 
       # MySQL doesn't allow default values on text fields
       if ActiveRecord::Base.connection.adapter_name == 'Mysql2'
@@ -59,17 +58,33 @@ def change
         t.text :metadata, default: '{}'
       end
 
+      if Statesman::Adapters::ActiveRecord.database_supports_partial_indexes?
+        t.boolean :most_recent, default: true, null: false
+      else
+        t.boolean :most_recent, default: true
+      end
+
       t.timestamps null: false
     end
 
     add_index :my_active_record_model_transitions,
               [:my_active_record_model_id, :sort_key],
               unique: true, name: "sort_key_index"
-    add_index :my_active_record_model_transitions,
-              [:my_active_record_model_id, :most_recent],
-              unique: true, where: "most_recent",
-              name: "index_my_active_record_model_transitions_"\
-                    "parent_most_recent"
+
+    if Statesman::Adapters::ActiveRecord.database_supports_partial_indexes?
+      add_index :my_active_record_model_transitions,
+                [:my_active_record_model_id, :most_recent],
+                unique: true,
+                where: "most_recent",
+                name: "index_my_active_record_model_transitions_"\
+                      "parent_most_recent"
+    else
+      add_index :my_active_record_model_transitions,
+                [:my_active_record_model_id, :most_recent],
+                unique: true,
+                name: "index_my_active_record_model_transitions_"\
+                      "parent_most_recent"
+    end
   end
 end
 # rubocop:enable MethodLength