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
1type OrderSummary = {2  id: string3  payment_collections: {4    id: string5    amount: number6  }[]7}8
9function summarizeOrder(order: OrderSummary) {10  // ...11}12
13const { data: orders } = await query.graph({14  entity: "order",15  fields: [16    "id",17    "payment_collections.id",18    "payment_collections.amount",19  ],20})21
22// TS2345 error23summarizeOrder(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
1type OrderSummary = {2  id: string3  payment_collections: ({4    id: string5    amount: number6  } | null)[] | null7}

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

Code
1function summarizeOrder(order: OrderSummary) {2  const paymentCollections = (3    order.payment_collections ?? []4  ).filter((pc) => pc !== null)5  // ...6}