Yes. It is necessary to suppose ideas (the forms of things, existing apart from the things themselves) in the divine mind because, since the world was not made by chance, but by God acting by His intellect (Q46, A1), there must exist in the divine mind a form to the likeness of which the world was made.
God does not understand things according to an idea existing outside Himself. Thus Aristotle (Metaph. ix) rejects the opinion of Plato, who held that ideas existed of themselves, and not in the intellect.
Although God knows Himself and all else by His own essence, yet His essence is the operative principle of all things, except of Himself. It has therefore the nature of an idea with respect to other things; though not with respect to Himself.
God is the similitude of all things according to His essence; therefore an idea in God is identical with His essence.