Reorganize array of hashes

have array of hashes it looks like

arr = [
  {"partner_name"=>"Bell", "publisher_name"=>"News - Calgary", "mn"=>"", "mid"=>415},
  {"partner_name"=>"Bell", "publisher_name"=>"News - Vancouver Island", "mn"=>"Module 2.0 ", "mid"=>4528},
  {"partner_name"=>"Bell", "publisher_name"=>"News - Atlantic", "mn"=>"Module 2.0 ", "mid"=>4531},
  {"partner_name"=>"Bell", "publisher_name"=>"News - Kitchener", "mn"=>"Module 2.0 ", "mid"=>4535},
  {"partner_name"=>"Bell", "publisher_name"=>"News - London", "mn"=>"Module 2.0 ", "mid"=>4536},

]

i have tried to do arr.group_by{|el|el['partner_name']}

i want to achieve this result

{
  partner:'Bell',
  publishers: [
    {name:'News - Vancouver Island'},
   modules:[
    {mn: val, mid: id_val},
    # ...
    {name:'News - Vancouver Island'
     modules:[
    {mn: val, mid: id_val},
    # ...
   ],},
    # ... and others
  ],
}

How can i do it?