Ggtcf

I have this datamodel:

type Item 
 id: ID! @unique
 title: String!
 description: String!
 user: User!
 pictures: [Picture]
 basePrice: Int!
 addons: [Addon]

I'm writing a query called parsedItem that takes the id from arguments and looks for the Item (using the default query for Item generated by Prisma), something like this:

 const where = id: args.id ;
 const item = await ctx.db.query.item( where , 
 `{
 id
 title
 ...

I need to show on the frontend a computed value: "dynamicPrice" it depends on the quantity of the Addons that the Item has.
e.g:
Item #1 has 3 addons, each addons has a value of $5. This calculated value should be

dynamicPrice = basePrice + 3 * 5

The Addon relation could change, so I need to compute this in every request the frontend makes.

I'd like so much to do something like:

item.dynamicPrice = item.basePrice + (item.addons.length * 5)

and return this item in the resolver, but this doesn't work. That throw an error:

"message": "Cannot query field "dynamicPrice" on type "Item"."
(when I try to query the Item from the frontend)

This error message makes me think: Should I create dynamicPrice as a field on the datamodel? Can I then populate this field in the query resolver? I know I can, but is this a good approach?

This is an example, I need to create more computed values for this Item model.

What is the best scalable solution/workaround for this simple use case?

You need create field resolver for dynamicPrice field at Item type. It will looks like that:

const resolvers = 
 Query: 
 parsedItem: (parent, args, ctx, info) => 
 ...
 
 ...
 ,
 Item: 
 dynamicPrice: parent => parent.basePrice + parent.addons.length * 5
 

More details you can find at A Guide to Common Resolver Patterns.