It is Mercurius job to group GraphQL types by handler instance

[oceanmotive]

Hello,
at the company I am working for,
we have GraphQL interface like Animal,
and seven GraphQL types which are implementation of Animal - Dog, Cat, Fish, ....
and we have dataloaders to load part of fields of every Animal
like

const dataloaderFunc = (...) => ...
{ // this is a dataloader definition which then will be loaded into mercurius plugin
Dog : dataloaderFunc,
Cat : dataloaderFunc,
Fish : dataloaderFunc,
...
}
I.e. for every Dog,Cat,Fish.. we want the same dataloader to be used

And we have a query which gives us an overview of all Animals, like
allAnimals() {
color : ...
voice : ... // other fields of that interface
}

What happens internally is that mercurius gets all Animals, they are of different types (Dog, Cat, Fish, ...),
group them by type, and pass each group to the respective dataloaderFunc.

So if we have in the resulte set 5 Dogs, 2 Cats and 3 Fish, there will be 3 calls of dataloaderFunc, each with the respective set.
What I am implying here is that (look at the above) I wanted for mix of Dogs,Cats,Fishes... to be passed into dataloaderFunc via single call.
It is not only the same expression( * : dataloaderFunc ), but also the same instance (dataloaderFunc).

Instead, I get something that goes against purpose of dataloaders (solve N+1 problem); I have got multiple calls of dataloaderFunc when they might be combined into one.

I think that when Mercurius starts, it might make a check of dataloaders to check what types should be grouped based on equality of the respective dataloaders.

[mcollina]

That case is uncommon: you have 3 entities that are essentially aliases of each other. We don't want to handle this automatically, because detecting this is pretty hard. You can always implement the dataloader pattern yourself, there is nothing really specific of our implementation.

[oceanmotive]

Why is it hard, you just compare dataloaderFunc for triple equality. That is the point: what is expressed in that literal is that any of those types should be resolved with the same dataloader. It is about a line or two in Mercurius code, and it might be done only once at boot up.

[oceanmotive]

Instead of all Mercurius users merging back those calls into one.

[mcollina]

Ok, feel free to send a PR, I'll evaluate it then.