title: Redux-toolkit createEntityAdapter
body: <p>The following code works as intended. The state gets populated using the createEntityAdapter.</p>
  <pre><code>import { createSlice, createAsyncThunk, createEntityAdapter } from '@reduxjs/toolkit';
  
  export const fetchUsers = createAsyncThunk('users/fetchUsers', async () =&gt; {
    const response = await fetch('https://mysite/wp-json/wp/v2/users');
    return response.json();
  });
  
  export const usersAdapter = createEntityAdapter();
  const initialState = usersAdapter.getInitialState({ status: 'idle' });
  
  export const usersSlice = createSlice({
    name: 'users',
    initialState,
    reducers: {},
    extraReducers: (builder) =&gt; {
      builder.addCase(fetchUsers.pending, (state) =&gt; {
        state.status = 'pending';
      });
      builder.addCase(fetchUsers.fulfilled, usersAdapter.upsertMany);
    },
  });
  
  export default usersSlice.reducer;
  </code></pre>
  <p>However, with the fulfilled promise, I also want to update the 'status' property of the state to 'idle'.</p>
  <p>When I change the fulfilled case to:</p>
  <pre><code>builder.addCase(fetchUsers.fulfilled, (state, action) =&gt; {
        usersAdapter.upsertMany;
        state.status = 'idle';
      });
  </code></pre>
  <p>And when I get add an argument the upsertMany</p>
  <pre><code>builder.addCase(fetchUsers.fulfilled, (state, action) =&gt; {
        usersAdapter.upsertMany(action.payload);
        state.status = 'idle';
      });
  </code></pre>
  <p>I get the error: Possible unhandled Promise Rejection (id: 0)</p>
  <p>Apart from the <strong>question how to make it work</strong>, could I ask one additional question?
  In the first code snippet (That one that works):</p>
  <pre><code>builder.addCase(fetchUsers.fulfilled, usersAdapter.upsertMany);
  </code></pre>
  <p>I understand that the addCase has two arguments: action type/creator (fetchUsers.fulfilled is the action creator here), and a reducer (usersAdapter.upsertMany in this case). Here fetchUsers returns the object, but <strong>how does usersAdapter.upsertMany know what to insert/update as payload?</strong> There's no argument there (eg. usersAdapter.upsertMany(action.payload). In all other reducers requiring some payload, there's always an argument (state, action) eg.</p>
  <pre><code>builder.addCase(incrementBy, (state, action) =&gt; state.counter += action.payload)
  </code></pre>