Use case for merge function in type policy?

I previously (in September 2020) asked this question on Apollo’s community chat and on Stack Overflow, but am reposting it here because it hasn’t been answered.

This question is about the cache introduced by Apollo 3.0. This update deprecated local queries in favour of “type policies” (or “field policies”), which include several tools for working with the cache, such as a merge function that can be defined for each GraphQL type to specify how incoming data should be merged with existing data in the cache.

I’m wondering which use cases are better suited for the merge function, versus writing a precalculated update value via writeQuery or writeFragment.

Let’s say I have an array with objects, and this array is property of another type. For example:

type Person {
  hobbies: [Hobby!]!

The application allows me to delete, add, and update elements in this array. Should hobbies make use of the merge function? Previously, I wrote this logic in local mutation resolvers, but these have been deprecated. I can easily mimic the old behaviour by defining a function for each of these resolvers, which contains the add/remove/update logic, and uses writeFragment to store the results.

I can see the benefit of using merge because (I think) it’s a lower layer of abstraction, but can it be used in the case of incoming hobbies array? As far as I can see, the way it would work would imply that we’d have to deduce the type of mutation by means of the incoming input. For example:

  • If the incoming array contains one less item (or only contains the deleted item), we did a delete operation, and we can assume that all other elements remained the same.
  • If the incoming array contains just one element and it is new, we did an add operation, and will merge this one element inside the array
  • If the incoming array contains the same number of elements (or has just one updated item), we must have updated one and must now figure out which one to replace (let’s assume that, due to the implementation, it is not possible to directly update a single Hobby, because other hobbies may be affected as well, e.g. due to non-overlapping time constraints).

This seems less elegant than mirroring the old approach where the new value for hobbies is calculated before calling writeFragment.

Is there a performance benefit to using merge? Are existing and non-affected items in hobbies kept in place when using merge, and overwritten when using writeFragment? Assuming the newly calculated data passed to writeFragment contains an array with shallow copies for the unmodified elements.

Thank you very much for clearing this up for me!

In the meantime, I implemented a merge function similar to what’s outlined in the Apollo docs. This function reuses as many items from the existing array as possible, and takes an item from incoming otherwise. Equality is checked based on object id, which is pretty much always present in my app.

The only (minor) downside to this approach seems to be that it doesn’t handle the case where items in an array are being changed, but such manipulations are done in separate mutations that don’t return an array, anyway.

For the sake of posterity, my implementation is as follows:

import type { FieldPolicy, Reference } from '@apollo/client'

 * Based on example in Apollo docs
 * @see
export const mergeArrayByIdFieldPolicy: FieldPolicy<Reference[]> = {
  // eslint-disable-next-line @typescript-eslint/default-param-last
  merge: (existing = [], incoming = [], { readField, mergeObjects }) => {
    const merged = [...incoming]
    const existingIds = => readField<UUID>('id', item))

    merged.forEach((item, index) => {
      const itemId = readField<UUID>('id', item)
      const existingIndex = existingIds.findIndex((id) => id === itemId)
      if (existingIndex !== -1) {
        merged[index] = mergeObjects(existing[existingIndex], merged[index])
    return merged

In my use case, I used the merge function for queries that use fetchMore throughout my app.
Instead of having to handle updating the cache per use of the query, I did it directly with the merge function.

True that I could simply create a generic component or hook to handle the fetchMore and writing to cache, but I went that route instead and it allowed me to test the feature.

1 Like

Not sure if this is exactly what you’re looking for, but the cache.modify method bypasses the merge function, which can be useful for situations like deleting an item from a list.