Let me explain:
1 - I take an object(example: bread) and put it in the shelf.
2 - I take a second object(example: can) and I want it not to be possible to put it in the same shelf but in a different one
3 - I take the bread again and put it next to the other and so on until the space is used up
I want for the same shelf I can put all the objects I create, but in one shelf the objects must all be of the same type.