Translating proof from Nat to Rat

I'm trying to use a CoQ/SSReflect proof using nat to prove a very similar statement in rat. The current proof status, within an Open Scope ring_scope, is

  (price bs i - price bs' i <= tnth bs i * ('ctr_ (sOi i) - 'ctr_ (sOi i')))%N
  → (price bs i)%:~R - (price bs' i)%:~R <=
    (value_per_click i)%:~R * (('ctr_ (sOi i))%:~R - ('ctr_ (sOi i'))%:~R)

and, using Set Printing All, it shows as

 forall
    _ : is_true
          (leq (subn (price bs i) (price bs' i))
             (muln (@nat_of_ord p (@tnth n bid bs i))
                (subn (@nat_of_ord q (@tnth k ctr cs (sOi i)))
                   (@nat_of_ord q (@tnth k ctr cs (sOi i')))))),
  is_true
    (@Order.le ring_display (Num.NumDomain.porderType rat_numDomainType)
       (@GRing.add rat_ZmodType
          (@intmul (GRing.Ring.zmodType rat_Ring) (GRing.one rat_Ring) (Posz (price bs i)))
          (@GRing.opp rat_ZmodType
             (@intmul (GRing.Ring.zmodType rat_Ring) (GRing.one rat_Ring) (Posz (price bs' i)))))
       (@GRing.mul rat_Ring
          (@intmul (GRing.Ring.zmodType rat_Ring) (GRing.one rat_Ring)
             (Posz (value_per_click i)))
          (@GRing.add (GRing.Ring.zmodType rat_Ring)
             (@intmul (GRing.Ring.zmodType rat_Ring) (GRing.one rat_Ring)
                (Posz (@nat_of_ord q (@tnth k ctr cs (sOi i)))))
             (@GRing.opp (GRing.Ring.zmodType rat_Ring)
                (@intmul (GRing.Ring.zmodType rat_Ring) (GRing.one rat_Ring)
                   (Posz (@nat_of_ord q (@tnth k ctr cs (sOi i')))))))))

I have been trying to use various rewrite such as ler_nat, PoszM, intrM, but with not much success. Could anyone provide me with some hints on how to proceed?

PS: I'm not able to provide a minimal working example, given I'm not exactly mastering what I'm doing here ;)

As you might have noticed, going from nat to rat there are two embeddings: first from nat to int, then from int to rat. The latter is a ring morphism, you can thus use generic morphism theorems such as rmorphM and rmorphB, in your case you could start with rewrite -!rmorphB -rmorphM ler_int.

The former embedding (Posz : nat -> int) however is not a ring morphism, you could still use PoszM indeed (Posz is multiplicative), but the major problem would be that Posz (m - n) != Posz m - Posz n in general (and the silent insertion of the coercion complicates matter here). So it would appear that you would need to postulate both (price bs' i <= price bs i)%N and 'ctr_ (sOi i') <= 'ctr_ (sOi i). However thanks to leq_subLR you can avoid the first hypothesis.

Here is a mock-up of your problem and a solution (it would have been nice to have the full context if you could not minimize). Assuming I reverse engineered the right types for price _ _ (thereafter abbreviated with p and p'), 'ctr _ _ (thereafter abbreviated with c and c') and value_per_click _ (abbrev v):

Lemma test (p p' v c c' : nat) : (c' <= c)%N -> (p - p' <= v * (c - c'))%N ->
  p%:~R - p'%:~R <= v%:~R * (c%:~R - c'%:~R) :> rat.
Proof.
rewrite leq_subLR => le_c'c le_pp'_vMcc'. (* Removing the first subn. *)
rewrite -!rmorphB -rmorphM ler_int. (* Changing rat goal into int goal. *)
by rewrite ler_subl_addl subzn. (* Changing int goal into nat goal. *)
(* The rest of the proof was actually carried out using conversion. *)
Qed.