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Usage

The firestoreMerge function allows for merging updates to a document within a Firestore collection. The primary use case is to update fields in a document without overwriting the entire document. This is especially useful for partial updates. Usage is straightforward: 
// Define a function to merge updates into a post
const mergePost = firestoreMerge('posts')

// Use the function to merge updates into a post
const postId = 'post-id-1'
await mergePost(postId, { title: 'Updated title' })

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Subcollections

firestoreMerge can also handle subcollections. To update a document in a subcollection, the collection path must include the subcollection path: 
// Define a function to merge updates into a post for a specific user
const userId = 'user-id-1'
const mergePostForUser = firestoreMerge(`users/${userId}/posts`)

const postId = 'post-id-1'
await mergePostForUser(postId, { title: 'Updated title' })

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Dynamic Paths

For more dynamic use cases, you can use a function to determine the collection path: 
// Define a function to merge updates into a post for a user
const mergePostForUser = firestoreMerge(({ userId }) => `users/${userId}/posts`)

const postId = 'post-id-1'
const userId = 'user-id-1'
await mergePostForUser(postId, { title: 'Updated title' }, { userId })

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Return Value

The firestoreMerge function returns a Promise that resolves to a DocumentReference of the merged document: 
const postId = 'post-id-1'
const ref = await mergePost(postId, { title: 'Updated title' })

// The value returned is a DocumentReference
console.log(ref.path) // outputs the path of the updated document

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Type Safety

When a type parameter is provided, the function ensures type safety for the data being merged: 
type Post = {
  title: string
  content: string
}

const mergePost = firestoreMerge<Post>('posts')

const postId = 'post-id-1'

// This merge is allowed as it matches the Post type
await mergePost(postId, { title: 'Updated title' })

// This merge is not allowed as 'views' is not part of the Post type
await mergePost(postId, { views: 100 })
Although type definitions help prevent mistakes, they are not a guarantee against invalid data. Ensure proper usage of the function for accurate data updates.

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Options

You can also include metadata when merging data by using the addMetadata option. 
// The mergePost function will now include metadata.
const mergePost = firestoreMerge<Post>('posts', { addMetadata: true })

await mergePost('post-id-1', { title: 'My Merged Post' })

// The post will have an updated 'metadata.timeUpdated' field.