Validates presense vs null false in Rails models/tables

I was playing around with Rails admin and I noticed something.

Attributes which are defined as below in model, counted as "Required" in Rails admin

 validates :user, presence: true

However, attributes which are defined as below in table (schema/migration) still counted as "Optional"

t.datetime "created_at",:null => false

I would have assumed that both of these are identical except perhaps the level from which the validation error pops up. Am I wrong or is this a Rails admin error? Are both of these ensuring that this field will be required for a successful field save or is there a difference?