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Query Result Not Assignable to Custom Type Error

If you get a type error similar to the following when you pass the result of query.graph or useQueryGraphStep to a function or variable that uses your own type:

Terminal
error TS2345: Argument of type '{ ...;
payment_collections: Maybe<Maybe<PaymentCollection>[]>; }'
is not assignable to parameter of type 'OrderSummary'.

Why this Error Occurred#

Medusa generates the types of Query results from the schema of your modules and their links. In these generated types, a to-many relation of a module link, such as an order's payment_collections, has the following type:

Code
Maybe<Maybe<PaymentCollection>[]>

Maybe<T> is a shorthand for T | null. So, TypeScript considers both the array and each of its items nullable.

If you define your own type for the retrieved data where the relation is a non-nullable array, TypeScript can't assign the Query result to it. For example:

Code
type OrderSummary = {
  id: string
  payment_collections: {
    id: string
    amount: number
  }[]
}

function summarizeOrder(order: OrderSummary) {
  // ...
}

const { data: orders } = await query.graph({
  entity: "order",
  fields: [
    "id",
    "payment_collections.id",
    "payment_collections.amount",
  ],
})

// TS2345 error
summarizeOrder(orders[0])

This error only affects type-checking. Your code works as expected at runtime, but the type error fails the build command.

How to Fix it#

Option 1: Remove Null Items from the Relation#

Map the Query result to your type, and filter out null items from the relation's array with a type predicate. In a workflow, perform the mapping in a transform function.

For example:

The type predicate pc is NonNullable<typeof pc> tells TypeScript that the filtered array no longer has null items, so the result matches your type.

Option 2: Accept Nullable Values in Your Type#

Alternatively, change your type to accept null for the relation and its items:

Code
type OrderSummary = {
  id: string
  payment_collections: ({
    id: string
    amount: number
  } | null)[] | null
}

Then, handle the null values in the code that uses the type. For example:

Code
function summarizeOrder(order: OrderSummary) {
  const paymentCollections = (
    order.payment_collections ?? []
  ).filter((pc) => pc !== null)
  // ...
}

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