make sure cached table name is a string. fixes #12582

activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb

@@ -20,7 +20,7 @@ def klass; @rhs_class_name.constantize; end
 
       def self.build(lhs_class, name, options)
         if options[:join_table]
-          KnownTable.new options[:join_table]
+          KnownTable.new options[:join_table].to_s
         else
           class_name = options.fetch(:class_name) {
             name.to_s.camelize.singularize

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -570,6 +570,13 @@ def test_consider_type
     assert !developer.special_projects.include?(other_project)
   end
 
+  def test_symbol_join_table
+    developer = Developer.first
+    sp = developer.sym_special_projects.create("name" => "omg")
+    developer.reload
+    assert_includes developer.sym_special_projects, sp
+  end
+
   def test_update_attributes_after_push_without_duplicate_join_table_rows
     developer = Developer.new("name" => "Kano")
     project = SpecialProject.create("name" => "Special Project")

activerecord/test/models/developer.rb

@@ -36,6 +36,10 @@ def find_least_recent
       end
 
   has_and_belongs_to_many :special_projects, :join_table => 'developers_projects', :association_foreign_key => 'project_id'
+  has_and_belongs_to_many :sym_special_projects,
+                          :join_table => :developers_projects,
+                          :association_foreign_key => 'project_id',
+                          :class_name => 'SpecialProject'
 
   has_many :audit_logs
   has_many :contracts