In his lucid, concise Medieval Trinitarian Thought from Aquinas to Ockham 
For Bonaventure, “the Father is the Father because he generates. The absolutely fundamental reason that the Father is established in being as the Father . . . is that he generates the Son.” This is an “emanation” account of the Father’s personality. The Father is the Person he is because of the emanation that proceeds from Him.
Thomas disagrees.
How, he asks, can the Father generate at all unless He is first constituted as a distinct person prior to generating the Son? For Thomas, Bonaventure’s position is nonsensical, implying that “the Father gives himself being through the action of generating.” Thomas says that there has to be an actor prior to the act, and so the “Father generates because he is the Father, and is the Father because of the opposition of the relations of paternity and filiation. The Father and the Son are constituted in being as an opposed pair, just as the relation account of personal distinction claims.”
Bonaventure, in his turn, disagrees with Thomas. Can Thomas apply the logic he offers to the Son? No: “the emanation of the Son from the Father, i.e., the generation, is precisely that which establishes the Son as a person distinct from the Father, and the relations filiation and paternity cannot come about before that.” Just as the Son is the person He is because generated by the Father, so the Father is the person He is by virtue of generating the Son.
In sum, “Aquinas, taking his point of departure in the view that an act can proceed only from a distinct individual, consistently places the constitution of the persons prior (conceptually speaking) to the active emanation of one person from another. For Aquinas, the relations take on an existence of their own ‘prior’ to the emanations of the persons. The persons are established as really distinct in God only by the opposition of relations, the personal acts follow only ‘after’ the persons are made distinct.” That’s not good, I think. We have individual persons conceptually fundamental, prior to the emanations that constitute them. It’s hard to see how Thomas can even sustain his relation-of-opposition view, for what can it mean for the Father to be Father prior (conceptually) to the emanation of the Son?
Bonaventure ends in similar conundrums. For him, the emanations are primary: “the emanations or origins of the persons, upon which are founded the relations of origin, account for the distinction between the persons, the distinction between the persons is in no way prior to the emanations.” But he still needs to meet Thomas’s challenge: If the Father generates, but is constituted as Father by that generation, who is generating? Who’s the implied actor of that action? Friedman summarizes Bonaventure’s answer thus: “at the level of our concepts of God, we must understand there to be a potentiality for generation in a type of ‘proto-Father’ (my term). It is from the proto-Father that the act of generation comes, and it is ‘after’ the act of generation that the proto-Father ‘becomes’ the Father.” To which one can only reply, Yeek!
Is there a way out? Perhaps something along the lines of an actualist ontology? Is the problem here the notion that we have to have a firm and settled “actor” to have an “act,” that both Thomas and Bonaventure are assuming a placid inactivity must precede the disturbance of activity? What happens if the being of the actor is eternal action? What happens if it disturbance all the way down? Barth says (for instance) of Christ: “His existence is act; that it is being in spontaneous actualisation. Primarily and supremely we have again to say actus purus , the actualisation of being in absolutely sovereign spontaneity, after the manner in which the Creator, God, actualises Himself, so that His life-action is identical with that of God Himself, His history with the divine history.” The Father, then, is His activity of generating the Son.