Crud plugin
Legend-State includes a syncedCrud
plugin that runs on top of synced
and encapsulates a lot of the behavior youâd use to sync with a CRUD backend.
You can use syncedCrud
directly or you can build a plugin for your backend on top of it. See the source of the Keel and Supabase plugins for examples of plugins built on top of syncedCrud
.
get and list
The crud plugin has two slightly different patterns depending on whether youâre using a get
or a list
action.
The behavior when using get
is:
- get: Observable value is the value returned from get
- create: If get returned null, then setting any value on the observable will create
- update: If get returned a value, then updating any value on the observable will update
- create: Setting the value to null or undefined, or calling
delete()
, will delete
The behavior when using list
is:
- list: Observable value is an object containing the listed values keyed by id
- create: Adding a new value to the object will will create
- update: Updating a child value will update it with the changed fields
- delete: Setting a child value to null or undefined, or calling
delete()
, will delete
The list
function expects an array of rows to be returned from you API.
The shape of the observable object returned from a list
can be changed with the as parameter, which supports three options:
object
: The default, an object keyed by the rowâsid
field.array
: Treat the result of a query as an arrayMap
: A Map, which can be more efficient for accessing rows by keyvalue
: Treat the result of a query as a single value like aget
create
The create
function is called whenever a new object is added to the observable. If you provide a fieldCreatedAt
then this is determined by whether the object has a value at that field. Otherwise itâs determined by whether the new value was previously undefined.
The returned value will be merged into the local value, applying any server defaults or created/updated times from the server value. See onSaved for more details.
update
If an element in the observable is updated it will call the update
function with the changed value. If youâve enabled the updatePartial
option then the value will include only the changed fields and the id
. Otherwise it will be the full changed object.
The returned value will be merged into the local value, applying any server defaults or created/updated times from the server value. See onSaved for more details.
delete
When an element is deleted from the observable, it will call the delete
function with the id
of the deleted element.
Alternatively if you do soft deletes, you can provide a fieldDeleted
option instead of delete
, and then it will call the update
function with that field set to true.
onSaved
When a value is saved to the server you may want it to apply changes back into the local observable. There are two ways to do that.
- onSavedUpdate: âcreatedUpdatedAtâ: This will save any fields ending in
["createdAt", "updatedAt", "created_at", "updated_at"]
back to the observable. This can be useful if your backend updates these values on the server. It also works if you have updated times for specific fields like ânoteUpdatedAtâ.
- onSaved: If you want more control over what fields are updated in your object you can do it manually with
onSaved
. Just return an object with the fields you want merged into the observable. Note that you can also just use this for side effects and not return anything.
subscribe
If your backend has a realtime feature, or if you want to poll periodically for changes, use subscribe
to set that up. This will be called only once after the first get
.
This can be used in two ways depending on how your backend works, updating with incoming data or simply triggering a refresh.
When the observable is no longer being observed it will call the returned unsubscribe function.
Sync only diffs
An optional but very useful feature is the changesSince: 'last-sync'
option. This can massively reduce badwidth usage when youâre persisting list results since it only needs to list changes since the last query. The way this works internally is basically:
- Save the maximum updatedAt to the local persistence
- In subsequent syncs or after refresh it will list by
updatedAt: lastSync + 1
to get only recent changes - The new changes will be merged into the observable
This has a few requirements to work correctly:
- Set the
fieldUpdatedAt
with a field that is automatically updated by your backend on save. It should not be set on the frontend because inaccurate user clocks might cause data to be missed. - Use soft deletes instead of deleting rows or include deleted rows in your list function. If the list function does not include rows deleted since the last update, the frontend will not know to delete them. You can enable this by adding a
deleted
field in your backend and setting thefieldDeleted
option.
All options
get
: Get a single value from the backendlist
: List an array of values from the backendcreate
: Create a single value on the backendupdate
: Update a single value on the backenddelete
: Delete a single value on the backendonSaved
: Update local value with remote dataonSavedUpdate
: Automatically update local value with created and updated timesfieldCreatedAt
: Set the field your backend uses for created timesfieldUpdatedAt
: Set the field your backend uses for updated timesfieldDeleted
: Set the field your backend uses for soft deletesupdatePartial
: Send only changed fields into update functionchangesSince
: âallâ | âlast-syncâ. Defaults to âallâ. âlast-syncâ syncs only diffsgenerateId
: Provide a function for creating row ids.subscribe
: Set up a realtime connection or pollingretry
: Options for retrying in case of error. Applies to both get and set.persist
: Options for persisting locally. See Persist and sync.debounceSet
: Debounce saved changes to reduce the number of updatesmode
: âsetâ | âassignâ | âmergeâ | âappendâ | âprependâ. How to apply incoming changes.transform
: Transform data as it loads in from the remote source or out as it saves to the remote source. You could use this to encrypt the data or transform it into some other format.waitFor
: A Promise or Observable to wait for before getting from remotewaitForSet
: A Promise or Observable to wait for before setting to remote