Correctly cast calculation results on PG

MySQL reports the column name as `"MAX(developer_id)"`. PG will report
it as `"max"`

4 files changed, 17 insertions, 2 deletions

diff --git a/Gemfile b/Gemfile
index f6fd0f3ac0..931c8e85bc 100644
--- a/Gemfile
+++ b/Gemfile
@@ -14,7 +14,7 @@ gem 'rack-cache', '~> 1.2'
 gem 'jquery-rails', '~> 4.0.0.beta2'
 gem 'coffee-rails', '~> 4.1.0'
 gem 'turbolinks'
-gem 'arel', '~> 6.0.0.beta2'
+gem 'arel', github: 'rails/arel'
 
 # require: false so bcrypt is loaded only when has_secure_password is used.
 # This is to avoid ActiveModel (and by extension the entire framework)
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 96b7313234..dc1f3c9457 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -386,6 +386,10 @@ module ActiveRecord
         type_map.lookup(sql_type)
       end
 
+      def column_name_for_operation(operation, node) # :nodoc:
+        node.to_sql
+      end
+
       protected
 
       def initialize_type_map(m) # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 8e209e1051..e472741660 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -393,6 +393,16 @@ module ActiveRecord
         super(oid)
       end
 
+      def column_name_for_operation(operation, node) # :nodoc:
+        OPERATION_ALIASES.fetch(operation) { operation.downcase }
+      end
+
+      OPERATION_ALIASES = { # :nodoc:
+        "maximum" => "max",
+        "minimum" => "min",
+        "average" => "avg",
+      }
+
       protected
 
         # Returns the version of the connected PostgreSQL server.
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index e20cc0e76d..c8ebb41131 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -253,7 +253,8 @@ module ActiveRecord
 
         select_value = operation_over_aggregate_column(column, operation, distinct)
 
-        column_alias = select_value.alias || select_value.to_sql
+        column_alias = select_value.alias
+        column_alias ||= @klass.connection.column_name_for_operation(operation, select_value)
         relation.select_values = [select_value]
 
         query_builder = relation.arel