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 when = {};

module.exports = { fields, when };

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 when object. It receives three parameters, the messages list itself, the actual event, and a mark object.

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. Afterwards you need to mark the event as handled by calling the mark.asDone function:

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

    mark.asDone();
  }
};

For more details, see handling events.

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

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

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

module.exports = { fields, when };

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!

# Creating the read model

# Defining fields

# Handling the sent event

# Handling the liked event

# Safety check

Creating the read model

Defining fields

Handling the sent event

Handling the liked event

Safety check