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DocumentationAdvanced guidesCreating an application from scratchCreating the read model

Creating the read model

To create the messages list, create a messages.js file within the lists directory:

$ touch chat/server/readModel/lists/messages.js

Where is the context?

In contrast to the write model the read model does not use contexts. This is because a read model can handle events from multiple contexts, so it may not be possible to assign a read model to a specific context.

Then, open the file and add the following base structure:

'use strict';

const fields = {};

const projections = {};

module.exports = { fields, projections };

For more details, see defining lists.

Defining fields

As you have decided while modeling that each message in the list of messages should have a text, a number of likes, and a timestamp, you need to define the appropriate fields:

const fields = {
  text: { initialState: '' },
  likes: { initialState: 0 },
  timestamp: { initialState: 0 }
};

Additionally, an id field is created automatically.

For more details, see defining fields.

Handling the sent event

The next question is how the list becomes filled with messages. For that you need to handle the events that have been published by the write model.

Whenever a message has been sent, add it to the list of messages, and set the text and timestamp to the data that are provided by the event. Add a communication.message.sent function to the projections object. It receives two parameters, the messages list itself and the actual event.

Add the message to the list by calling its add function. You do not need to set the id field, as it gets automatically populated using the aggregate's id that is given in the event:

const projections = {
  'communication.message.sent' (messages, event) {
    messages.add({
      text: event.data.text,
      timestamp: event.metadata.timestamp
    });
  }
};

For more details, see defining projections.

Handling the liked event

Handling the liked event is basically the same as handling the sent event. The only difference is that now you need to update an existing message instead of adding a new one. Again, add an event handler, but this time call the list's update function:

const projections = {
  // ...
  'communication.message.liked' (messages, event) {
    messages.update({
      where: { id: event.aggregate.id },
      set: {
        likes: event.data.likes
      }
    });
  }
};

Safety check

Before you proceed, make sure that your list looks like this:

'use strict';

const fields = {
  text: { initialState: '' },
  likes: { initialState: 0 },
  timestamp: { initialState: 0 }
};

const projections = {
  'communication.message.sent' (messages, event) {
    messages.add({
      text: event.data.text,
      timestamp: event.metadata.timestamp
    });
  },

  'communication.message.liked' (messages, event) {
    messages.update({
      where: { id: event.aggregate.id },
      set: {
        likes: event.data.likes
      }
    });
  }
};

module.exports = { fields, projections };

Yay, congratulations!

You have created your first read model, and clients can read and observe it in real-time!

Now we are ready for creating the client, so let's go ahead!