Rails 6.2.0.rc2 compatibility

Rails 6.2.0.rc2 changed join order when combining Arel and Rails joins.
Worked around by using only Arel joins in the affected query.
See rails/rails#36761

Fixes #822

Gemfile

@@ -2,7 +2,7 @@
 
 source 'https://rubygems.org'
 
-gem 'rails', '~> 6.0.0.rc1'
+gem 'rails', '~> 6.0.0.rc2'
 gem 'rails-i18n', '~> 6.0.0.beta1'
 
 # https://github.com/rails/rails/blob/v6.0.0.rc1/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L12

app/models/thredded/messageboard.rb

@@ -128,14 +128,16 @@ def unread_topics_counts(user:, topics_scope: Thredded::Topic.all)
         messageboards = arel_table
         read_states = Thredded::UserTopicReadState.arel_table
         topics = topics_scope.arel_table
-        joins(:topics).merge(topics_scope).joins(
-          messageboards.outer_join(read_states).on(
-            messageboards[:id].eq(read_states[:messageboard_id])
-              .and(read_states[:postable_id].eq(topics[:id]))
-              .and(read_states[:user_id].eq(user.id))
-              .and(read_states[:unread_posts_count].eq(0))
-          ).join_sources
-        ).group(messageboards[:id]).pluck(
+        joins(
+          messageboards.join(topics)
+            .on(topics[:messageboard_id].eq(messageboards[:id]))
+            .outer_join(read_states).on(
+              messageboards[:id].eq(read_states[:messageboard_id])
+                .and(read_states[:postable_id].eq(topics[:id]))
+                .and(read_states[:user_id].eq(user.id))
+                .and(read_states[:unread_posts_count].eq(0))
+            ).join_sources
+        ).merge(topics_scope).group(messageboards[:id]).pluck(
           :id,
           Arel::Nodes::Subtraction.new(topics[:id].count, read_states[:id].count)
         ).to_h

lib/thredded/db_tools.rb

@@ -9,13 +9,14 @@ class << self
       def migrate(paths:, quiet:, &filter)
         verbose_was = ActiveRecord::Migration.verbose
         ActiveRecord::Migration.verbose = !quiet
-        migrate = -> {
-          if Rails::VERSION::STRING >= '5.2'
-            ActiveRecord::MigrationContext.new(paths).migrate(nil, &filter)
+        migrate =
+          if Rails.gem_version >= Gem::Version.new('6.0.0.rc2')
+            -> { ActiveRecord::MigrationContext.new(paths, ActiveRecord::SchemaMigration).migrate(nil, &filter) }
+          elsif Rails::VERSION::STRING >= '5.2'
+            -> {ActiveRecord::MigrationContext.new(paths).migrate(nil, &filter) }
           else
-            ActiveRecord::Migrator.migrate(paths, &filter)
+            -> { ActiveRecord::Migrator.migrate(paths, &filter) }
           end
-        }
         if quiet
           silence_active_record(&migrate)
         else

spec/gemfiles/rails_6_0.gemfile

@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 gemspec path: '../../'
 eval_gemfile '../../shared.gemfile'
 
-gem 'rails', '~> 6.0.0.rc1'
+gem 'rails', '~> 6.0.0.rc2'
 gem 'rails-i18n', '~> 6.0.0.beta1'
 
 # https://github.com/rails/rails/blob/v6.0.0.rc1/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L12