How to use dependent: :destroy in rails?

I have 2 models as describes below.

class EmpGroup < ActiveRecord::Base
  belongs_to :user
  has_many :emp_group_members, dependent: :destroy
end

and

class EmpGroupMember < ActiveRecord::Base
  belongs_to :emp_group
  belongs_to :user
end

now the problem is whenever I tried to destroy a group then I received a error as below.

PG::ForeignKeyViolation: ERROR:  update or delete on table "emp_groups" violates foreign key constraint "fk_rails_bd68440021" on table "emp_group_members"
DETAIL:  Key (id)=(1) is still referenced from table "emp_group_members".

What I'm missing?

Thanks.