3 files changed, 33 insertions, 7 deletions
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 1f9dbdc3d4..afab793a0c 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -179,14 +179,13 @@ module ActiveRecord
     def pluck(column_name)
       if column_name.is_a?(Symbol) && column_names.include?(column_name.to_s)
         column_name = "#{connection.quote_table_name(table_name)}.#{connection.quote_column_name(column_name)}"
-      else
-        column_name = column_name.to_s
       end
 
-      relation = clone
-      relation.select_values = [column_name]
-      klass.connection.select_all(relation.arel).map! do |attributes|
-        klass.type_cast_attribute(attributes.keys.first, klass.initialize_attributes(attributes))
+      result = klass.connection.exec_query(select(column_name).to_sql)
+      last_column = result.columns.last
+
+      result.map do |attributes|
+        klass.type_cast_attribute(last_column, klass.initialize_attributes(attributes))
       end
     end
 
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 08cfe4f70d..cdf18f8080 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -254,6 +254,7 @@ module ActiveRecord
       values = @klass.connection.distinct("#{@klass.connection.quote_table_name table_name}.#{primary_key}", orders)
 
       relation = relation.dup.select(values)
+      relation.uniq_value = nil
 
       id_rows = @klass.connection.select_all(relation.arel, 'SQL', relation.bind_values)
       ids_array = id_rows.map {|row| row[primary_key]}
diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index b8e384c2f3..90f2ac3cde 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -27,7 +27,7 @@ module ActiveRecord
         merge_relation_method(merged_relation, method, value) if value.present?
       end
 
-      merged_relation.joins_values += r.joins_values
+      merge_joins(merged_relation, r)
 
       merged_wheres = @where_values + r.where_values
 
@@ -146,6 +146,32 @@ module ActiveRecord
 
     private
 
+      def merge_joins(relation, other)
+        values = other.joins_values
+        return if values.blank?
+
+        if other.klass == relation.klass
+          relation.joins_values += values
+        else
+          joins_dependency, rest = values.partition do |join|
+            case join
+            when Hash, Symbol, Array
+              true
+            else
+              false
+            end
+          end
+
+          join_dependency = ActiveRecord::Associations::JoinDependency.new(
+            other.klass,
+            joins_dependency,
+            []
+          )
+
+          relation.joins_values += join_dependency.join_associations + rest
+        end
+      end
+
       def merge_relation_method(relation, method, value)
         relation.send(:"#{method}_values=", relation.send(:"#{method}_values") + value)
       end