I'm doing something on divine ideas in Malebranche and the scholastics, so I thought I'd practice my patchy Latin a bit by translating what Aquinas says on the matter. The following is ST 1.15.1. The Latin is here; the Dominican Fathers translation is here; Freddoso's translation is here (PDF). As always, this translation is rough.
After consideration of God's knowledge, it remains to consider ideas. And on this three things are asked. First, if there are ideas. Second, whether they are many or only one. Third, whether there are ideas of all things known by God.
We proceed to the first in this way.
 It seems that there are no ideas. For Dionysius says (Div. Nom. 7) that God does not know a thing according to an idea. But ideas are not posited for anything but to be that throuch which a thing is known. Therefore there are not ideas.
 Further, God knows all things in Himself, as is said above. But He does not know Himself by idea. Therefore nothing else either.
 Further, an idea is posited as a principle of knowing and acting. But the divine esence suffices the principle of knowing and acting on all things. Therefore it is not necessary to posit ideas.
But to the contrary is what Augustine says (Quaest. LXXXIII), "Such is the power constituting ideas, that, unless they are understood, nobody is able to be wise."
I respond that it must be said that it is necessary to posit ideas in the divine mind. For the Greek 'idea' is called 'form' in Latin; hence by 'ideas' is understood the forms of all things, existing apart from things themselves. But the form of anything existing apart from it can be for two things: Either as being the exemplar of the thing of which it is called the form, or as the principle of knowing it [principium cognitionis ipsius], according to which the form of the knowable is said to be in the knower. In either way it is necessary to posit ideas, which is clear in this way: It is necesary for the form to be the end of any generation whatsoever in all thinsg that are not generated by chance [a casu]. But an agent does not act according to a form, save inasmuch as a similitude of the form is in him, which may occur in two ways [quod quidem contingit dupliciter]. For in some agents the form of the thing to be made pre-exists according to natural being, as in those that act by nature, as when man generates man and fire fire; in some it is instead according to intelligible being, as in those that act by understanding [per intellectum], as when the similitude of a house pre-exists in the mind of the builder. And this can be called the idea of the house, because the artificer intends to assimilate the house to the form which the mind conceives. Since, therefore, the world is not made by chance [casu], but is made by God through active understanding [per intellectum agente], as is seen below, it is necessary that there be in the divine mind forms, to the likeness of which the world is made. In this consists the nature [ratio] of an idea.
Therefore to the first it must be said that God does not understand the thing according to an idea existing outside himself. Thus Aristotle refutes the opinion of Plato on ideas, who posited them as existing per se, not in the understanding.
To the second it must be said that, although God by essence knows Himself and all things, His essence is the operative principle of other things, but not Himself, and it therefore has the nature of an idea with respect to others, but not with respect to God Himself.
To the third it must be said that God according to His essence is the similitude of all things. Hence an idea in God is nothing other than the essence of God.