Is this a bug in Rails validations or am I doing something wrong? -

this involves validations on join table, validating activerecord on either side of join against each other. seems not behave expected, allowing violation of validation.

i want allow users able belong groups, (or groups users, it's many-to-many). user's company must match group's company. hence, usergroup looks this:

class usergroup < activerecord::base   belongs_to :user   belongs_to :group    validate :group_company_matches_user_company     private   def group_company_matches_user_company     if user.company != group.company       self.errors.add(:group, "company must match user company")     end   end end 

now here test showing failure of validation:

test 'validation failure example'     group = groups(:default)     user = users(:manager)      #default user , group have same company in db     assert_equal user.company, group.company      #create 2nd company     company2 = user.company.dup     assert_difference 'company.count', 1       company2.save!     end      #set group's company new one, verify user , group's company don't match     group.company = company2         assert_not_equal user.company, group.company      #warning!!! passes , creates new usergroup. though violates     #the usergroup validation     assert_difference 'usergroup.count', 1       group.users << user     end      #what when save group db?     #no issues.     group.save      #this fail. have saved usergroup who's user.company != group.company     #despite validation requires otherwise     usergroup.all.each |ug|       assert_equal ug.user.company.id, ug.group.company.id     end   end 

using collection << object tl:dr bypasses validation

adds 1 or more objects collection setting foreign keys collection’s primary key. note operation instantly fires update sql without waiting save or update call on parent object.