Fix failing to detect encoding with JDBC MySQL adapter

Also switching JRuby CI to use MySQL instead of SQLite, because of frequent '[SQLITE_BUSY]' error

Author: mshibuya

.github/workflows/test.yml

@@ -52,7 +52,7 @@ jobs:
           - ruby: jruby
             gemfile: gemfiles/rails_6.1.gemfile
             orm: active_record
-            adapter: sqlite3
+            adapter: mysql2
             asset: sprockets
           - ruby: jruby
             gemfile: gemfiles/rails_6.1.gemfile

lib/rails_admin/adapters/active_record.rb

@@ -80,7 +80,11 @@ def encoding
         when 'postgresql'
           ::ActiveRecord::Base.connection.select_one("SELECT ''::text AS str;").values.first.encoding
         when 'mysql2'
-          ::ActiveRecord::Base.connection.raw_connection.encoding
+          if RUBY_ENGINE == 'jruby'
+            ::ActiveRecord::Base.connection.select_one("SELECT '' AS str;").values.first.encoding
+          else
+            ::ActiveRecord::Base.connection.raw_connection.encoding
+          end
         when 'oracle_enhanced'
           ::ActiveRecord::Base.connection.select_one('SELECT dummy FROM DUAL').values.first.encoding
         else

spec/rails_admin/abstract_model_spec.rb

@@ -112,7 +112,6 @@
       end
 
       it 'lists elements within outbound limits' do
-        pending('Due to the JRuby SQLite3 datetime boundary issue') if RUBY_ENGINE == 'jruby' && CI_ORM == :active_record
         expect(@abstract_model.all(filters: {'datetime_field' => {'1' => {v: ['', '2012-01-02T00:00:00', '2012-01-03T23:59:59'], o: 'between'}}}).count).to eq(2)
         expect(@abstract_model.all(filters: {'datetime_field' => {'1' => {v: ['', '2012-01-02T00:00:00', '2012-01-03T12:00:00'], o: 'between'}}}).count).to eq(1)
         expect(@abstract_model.all(filters: {'datetime_field' => {'1' => {v: ['', '2012-01-03T12:00:00', ''], o: 'between'}}}).count).to eq(2)