Document caveats with update operations

Author: markerikson

docs/api/createEntityAdapter.md

@@ -290,6 +290,14 @@ const bookIds = simpleSelectors.selectIds(store.getState().books)
 const allBooks = globalizedSelectors.selectAll(store.getState())
 ```
 
+## Notes
+
+### Applying Multiple Updates
+
+If `updateMany()` is called with multiple updates targeted to the same ID, they will be merged into a single update, with later updates overwriting the earlier ones.
+
+For both `updateOne()` and `updateMany()`, changing the ID of one existing entity to match the ID of a second existing entity will cause the first to replace the second completely.
+
 ## Examples
 
 Exercising several of the CRUD methods and selectors:

src/entities/unsorted_state_adapter.ts

@@ -110,6 +110,8 @@ export function createUnsortedStateAdapter<T>(selectId: IdSelector<T>): any {
       if (update.id in state.entities) {
         // If there are multiple updates to one entity, merge them together
         updatesPerEntity[update.id] = {
+          // Spreads ignore falsy values, so this works even if there isn't
+          // an existing update already at this key
           ...updatesPerEntity[update.id],
           ...update
         }