What are non-null constraints in GraphQL?
Mian Basam AhmadOverview
All types in the GraphQL system are nullable by default. This means that a type like integer can not only take a number, including 1, 2, 3,..., as its value but also take null, which means no values. At the same time, GraphQL also allows us to make any type non-null. This means that the types we make non-null will never return null. We always provide some values to the non-null types in GraphQL.
When to use
When we want to ensure that a particular field always exists, we use non-null constraints. It is important to remember that using a non-null field may make it more difficult to extend our GraphQL schema.
How non-null works
Let’s look at an example. In the following schema, we have a Hotel type with a hotelLocation field that refers to a Location type:
type Hotel {
hotelName: String!
rating: Int!
hotelLocation: Location!
}
type Location {
address: String!
}
type Query {
hotel(query: String): [Hotel]
}Non-null fields are those that have an exclamation mark (!) next to their type. When we query these fields, they will not return a null value. Here’s a query:
hotel {
hotelName
hotelLocation {
address
}
}
We can get the following outcomes:
// no restaurants exist
{ hotels: null }
// all data present
{
hotels: [{
hotelName: "pearl continental",
hotelLocation: {
address: "Street 85, San Francisco"
}
}]
}However, we will not be able to obtain the following outcomes:
// hotelName or hotelLocation not be null
// this result would be incorrect
{
hotel: [{
hotelName: null,
hotelLocation: null
}]
}To ensure the structure of the response, GraphQL may also ensure the presence of specific fields when queried.
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