8. The useMutation hook
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💻 Mutation in client-land

Playing with the Explorer was fun, but we do need to run the mutation from user input in our app. So let's move over to client land and send off our mutation using a new hook!

We want to update the number of views just before we navigate from the homepage to the track page. This navigation is happening inside our TrackCard component.

Inside the client/src/containers folder, let's open up the track-card.js file.

At the top, let's start importing from the @apollo/client package. We'll need gql, because we'll be using that same string template literal for our mutation, and we'll also need the useMutation hook to send our mutation to our server.

import {gql, useMutation} from '@apollo/client';

Next, let's make a new variable to hold our mutation called INCREMENT_TRACK_VIEWS, setting it to the gql template literal and adding backticks (`). Inside the backticks, we'll paste the mutation we built previously in Studio, and add a comment to explain what this mutation is for.

/**
* Mutation to increment a track's number of views
* (exported for tests)
*/
export const INCREMENT_TRACK_VIEWS = gql`
mutation IncrementTrackViewsMutation($incrementTrackViewsId: ID!) {
incrementTrackViews(id: $incrementTrackViewsId) {
code
success
message
track {
id
numberOfViews
}
}
}
`;

🎣 The useMutation hook

Because this is a mutation and not a query, we won't be using the useQuery hook we're familiar with. Instead we'll switch to the useMutation hook.

Inside the TrackCard component, we'll start off by calling the hook. It takes in the mutation we set up earlier, INCREMENT_TRACK_VIEWS, as the first parameter.

The second parameter is an object with a variables key. Here, we'll add the incrementTrackViewsId variable and set it to the id of the track we're navigating to. This id has already been destructured for us at the top from the track prop.

useMutation(INCREMENT_TRACK_VIEWS, {
variables: {incrementTrackViewsId: id}
});

Now, here's a twist: unlike with useQuery, calling useMutation doesn't actually execute the mutation automatically!

Instead, the useMutation hook returns an array with two elements, which we'll start to destructure here.

const [incrementTrackViews] = useMutation(INCREMENT_TRACK_VIEWS, {
variables: {incrementTrackViewsId: id}
});

The first element is the mutate function we'll use to actually run the mutation later on. We'll call it incrementTrackViews. The second element is an object with information about the mutation: loading, error and data. This component doesn't need it, so we don't have to extract it.

👆🏽Setting up the onClick

When do we want to run our mutate function? When the user clicks on the card!

Let's add an onClick prop to the CardContainer component and configure it to call our mutate function, incrementTrackViews.

<CardContainer
to={`/track/${id}`}
onClick={incrementTrackViews}
>
Loading...

Which of these are differences between the useQuery and useMutation hooks?

1️⃣ One more thing…

One last thing—let's add a console log to check the mutation response when it's completed.

To do this, let's go back to where we set up our useMutation hook and add another property to our options object. The onCompleted property is a callback function that will run when the mutation successfully completes, and it has access to the response that comes back. We'll log the response to the browser console.

const [incrementTrackViews] = useMutation(INCREMENT_TRACK_VIEWS, {
variables: {incrementTrackViewsId: id},
// to observe what the mutation response returns
onCompleted: data => {
console.log(data);
}
});

Our client app is ready to send off this mutation to the server! Let's see the results of our journey in the last lesson!

Code Challenge!

Use the useMutation hook to send the ASSIGN_SPACESHIP_MUTATION mutation to the server. It takes 2 variables: spaceshipId and missionId. Destructure the mutate function (call it assignSpaceship), as well as the loading, error and data properties from the return array of the hook.

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