Apollo Client normalizes all of your data so that if any data you previously fetched from your GraphQL server is updated in a later data fetch from your server then your data will be updated with the latest truth from your server.
This normalization process is constantly happening behind the scenes when you call
watchQuery but this process is often not enough to describe the updates to your data model as the result of a mutation. For example, if you wanted to add an item to the end of an array fetched by one of your queries. You also might want to read data from the normalized Apollo Client store at a specific id without making another GraphQL server fetch.
To interact directly with your data in the Apollo Client store you may use the methods
writeFragment that are accessible from the
ApolloClient class. This article will teach you how to use these methods to control your data.
If you would like a better understanding of the data normalization process then we recommend reading the 'How it works' documentation article. Knowledge around how Apollo Client works is not a prerequisite for using the methods described here, but it may be helpful.
All of the methods we will discuss can be called from the
ApolloClient class. Any code demonstration in this article will assume that we have already initialized an instance of
ApolloClient and that we have imported the
gql tag from
You can read more about Caching here
Being able to read and write to the Apollo cache from anywhere in your application gives you a lot of power over your data. However, there is one place where we most often want to update our cached data: after a mutation. As such, Apollo Client has optimized the experience for updating your cache with the read and write methods after a mutation with the
update function. Let us say that we have the following GraphQL mutation:
We may also have the following GraphQL query:
At the end of our mutation we want our query to include the new todo like we had sent our
TodoAppQuery a second time after the mutation finished without actually sending the query. To do this we can use the
update function provided as an option of the
Apollo.mutate method. To update your cache with the mutation just write code that looks like:
proxy argument is an instance of
DataProxy has the same for methods that we just learned exist on the Apollo Client:
writeFragment. The reason we call them on a
proxy object here instead of on our
client instance is that we can easily apply optimistic updates (which we will demonstrate in a bit). The
proxy object also provides an isolated transaction which shields you from any other mutations going on at the same time, and the
proxy object also batches writes together until the very end.
If you provide an
optimisticResponse option to the mutation then the
update function will be run twice. Once immediately after you call
apollo.mutate with the data from
optimisticResponse. After the mutation successfully executes against the server the changes made in the first call to
update will be rolled back and
update will be called with the actual data returned by the mutation and not just the optimistic response.
Putting it all together:
As you can see the
update function on
apollo.mutate provides extra change management functionality specific to the use case of a mutation while still providing you the powerful data control APIs that are available on
update function is not a good place for side-effects as it may be called multiple times. Also, you may not call any of the methods on