Queries and Mutations

On this page, you'll learn in detail about how to query a GraphQL server.

Fields #

At its simplest, GraphQL is about asking for specific fields on objects. Let's start by looking at a very simple query and the result we get when we run it:

You can see immediately that the query has exactly the same shape as the result. This is essential to GraphQL, because you always get back what you expect, and the server knows exactly what fields the client is asking for.

The field name returns a String type, in this case the name of the main hero of Star Wars, "R2-D2".

Oh, one more thing - the query above is interactive. That means you can change it as you like and see the new result. Try adding an appearsIn field to the hero object in the query, and see the new result.

In the previous example, we just asked for the name of our hero which returned a String, but fields can also refer to Objects. In that case, you can make a sub-selection of fields for that object. GraphQL queries can traverse related objects and their fields, letting clients fetch lots of related data in one request, instead of making several roundtrips as one would need in a classic REST architecture.

Note that in this example, the friends field returns an array of items. GraphQL queries look the same for both single items or lists of items, however we know which one to expect based on what is indicated in the schema.

Arguments #

If the only thing we could do was traverse objects and their fields, GraphQL would already be a very useful language for data fetching. But when you add the ability to pass arguments to fields, things get much more interesting.

In a system like REST, you can only pass a single set of arguments - the query parameters and URL segments in your request. But in GraphQL, every field and nested object can get its own set of arguments, making GraphQL a complete replacement for making multiple API fetches. You can even pass arguments into scalar fields, to implement data transformations once on the server, instead of on every client separately.

Arguments can be of many different types. In the above example, we have used an Enumeration type, which represents one of a finite set of options (in this case, units of length, either METER or FOOT). GraphQL comes with a default set of types, but a GraphQL server can also declare its own custom types, as long as they can be serialized into your transport format.

Read more about the GraphQL type system here.

Aliases #

If you have a sharp eye, you may have noticed that, since the result object fields match the name of the field in the query but don't include arguments, you can't directly query for the same field with different arguments. That's why you need aliases - they let you rename the result of a field to anything you want.

In the above example, the two hero fields would have conflicted, but since we can alias them to different names, we can get both results in one request.

Fragments #

Let's say we had a relatively complicated page in our app, which let us look at two heroes side by side, along with their friends. You can imagine that such a query could quickly get complicated, because we would need to repeat the fields at least twice - one for each side of the comparison.

That's why GraphQL includes reusable units called fragments. Fragments let you construct sets of fields, and then include them in queries where you need to. Here's an example of how you could solve the above situation using fragments:

You can see how the above query would be pretty repetitive if the fields were repeated. The concept of fragments is frequently used to split complicated application data requirements into smaller chunks, especially when you need to combine lots of UI components with different fragments into one initial data fetch.

Variables #

So far, we have been writing all of our arguments inside the query string. But in most applications, the arguments to fields will be dynamic: For example, there might be a dropdown that lets you select which Star Wars episode you are interested in, or a search field, or a set of filters.

It wouldn't be a good idea to pass these dynamic arguments directly in the query string, because then our client-side code would need to dynamically manipulate the query string at runtime, and serialize it into a GraphQL-specific format. Instead, GraphQL has a first-class way to factor dynamic values out of the query, and pass them as a separate dictionary. These values are called variables.

When we start working with variables, we need to do three things:

  1. Replace the static value in the query with $variableName
  2. Declare $variableName as one of the variables accepted by the query
  3. Pass variableName: value in the separate, transport-specific (usually JSON) variables dictionary

Here's what it looks like all together:

Now, in our client code, we can simply pass a different variable rather than needing to construct an entirely new query. This is also in general a good practice for denoting which arguments in our query are expected to be dynamic - we should never be doing string interpolation to construct queries from user-supplied values.

Variable definitions #

The variable definitions are the part that looks like ($episode: Episode) in the query above. It works just like the argument definitions for a function in a typed language. It lists all of the variables, prefixed by $, followed by their type, in this case Episode.

All declared variables must be either scalars, enums, or input object types. So if you want to pass a complex object into a field, you need to know what input type that matches on the server. Learn more about input object types on the Schema page.

Variable definitions can be optional or required. In the case above, since there isn't an ! next to the Episode type, it's optional. But if the field you are passing the variable into requires a non-null argument, then the variable has to be required as well.

To learn more about the syntax for these variable definitions, it's useful to learn the GraphQL schema language. The schema language is explained in detail on the Schema page.

Operation name #

One thing we also saw in the example above is that our query has acquired an operation name. Up until now, we have been using a shorthand syntax where we omit both the query keyword and the query name, but in production apps it's useful to use these to make our code less ambiguous.

Think of this just like a function name in your favorite programming language. For example, in JavaScript we can easily work only with anonymous functions, but when we give a function a name, it's easier to track it down, debug our code, and log when it's called. In the same way, GraphQL query and mutation names, along with fragment names, can be a useful debugging tool on the server side to identify different GraphQL requests.

Directives #

We discussed above how variables enable us to avoid doing manual string interpolation to construct dynamic queries. Passing variables in arguments solves a pretty big class of these problems, but we might also need a way to dynamically change the structure and shape of our queries using variables. For example, we can imagine a UI component that has a summarized and detailed view, where one includes more fields than the other.

Let's construct a query for such a component:

Try editing the variables above to instead pass true for withFriends, and see how the result changes.

We needed to use a new feature in GraphQL called a directive. A directive can be attached to a field or fragment inclusion, and can affect execution of the query in any way the server desires. The core GraphQL specification includes exactly two directives, which must be supported by any spec-compliant GraphQL server implementation:

  • @include(if: Boolean) Only include this field in the result if the argument is true.
  • @skip(if: Boolean) Skip this field if the argument is true.

Directives can be useful to get out of situations where you otherwise would need to do string manipulation to add and remove fields in your query. Server implementations may also add experimental features by defining completely new directives.

Mutations #

Most discussions of GraphQL focus on data fetching, but any complete data platform needs a way to modify server-side data as well.

In REST, any request might end up causing some side-effects on the server, but by convention it's suggested that one doesn't use GET requests to modify data. GraphQL is similar - technically any query could be implemented to cause a data write. However, it's useful to establish a convention that any operations that cause writes should be sent explicitly via a mutation.

Just like in queries, if the mutation field returns an object type, you can ask for nested fields. This can be useful for fetching the new state of an object after an update. Let's look at a simple example mutation:

Note how createReview field returns the stars and commentary fields of the newly created review. This is especially useful when mutating existing data, for example, when incrementing a field, since we can mutate and query the new value of the field with one request.

You might also notice that, in this example, the review variable we passed in is not a scalar. It's an input object type, a special kind of object type that can be passed in as an argument. Learn more about input types on the Schema page.

Multiple fields in mutations #

A mutation can contain multiple fields, just like a query. There's one important distinction between queries and mutations, other than the name:

While query fields are executed in parallel, mutation fields run in series, one after the other.

This means that if we send two incrementCredits mutations in one request, the first is guaranteed to finish before the second begins, ensuring that we don't end up with a race condition with ourselves.

Inline Fragments #

Like many other type systems, GraphQL schemas include the ability to define interfaces and union types. Learn about them in the schema guide.

If you are querying a field that returns an interface or a union type, you will need to use inline fragments to access data on the underlying concrete type. It's easiest to see with an example:

In this query, the hero field returns the type Character, which might be either a Human or a Droid depending on the episode argument. In the direct selection, you can only ask for fields that exist on the Character interface, such as name.

To ask for a field on the concrete type, you need to use an inline fragment with a type condition. Because the first fragment is labeled as ... on Droid, the primaryFunction field will only be executed if the Character returned from hero is of the Droid type. Similarly for the height field for the Human type.

Named fragments can also be used in the same way, since a named fragment always has a type attached.

Meta fields #

Given that there are some situations where you don't know what type you'll get back from the GraphQL service, you need some way to determine how to handle that data on the client. GraphQL allows you to request __typename, a meta field, at any point in a query to get the name of the object type at that point.

In the above query, search returns a union type that can be one of three options. It would be impossible to tell apart the different types from the client without the __typename field.

GraphQL services provide a few meta fields, the rest of which are used to expose the Introspection system.

Continue Reading →Schemas and Types