I think we need to see one your kid/sport model and then the new/ create method in the controller...
You should not have a id column btw in the kids_sports table.. .
This should be your up task:
def self.up create_table :kids_sports, :id => false do |t| t.integer :kid_id t.integer :sport_id end add_index :kids_sports, [:kid_id] add_index :kids_sports, [:sport_id] end
