{
 id_a => 1
 id_b => 2
 value_x => 20
 value_y => 30
}
{
 id_a => 3
 id_b => 4
 value_x => 40
 value_y => 50
}
{
 id_a => 1
 id_b => 4
 value_x => 100
 value_y => 200
}