[Question] ManyToMany relationship saving data in merge table

[galalaly]

I have a User Model with Many to many relationship with an Activity model.

The joint/merge table contains extra data. When I run the following code, only the user_id and the activity_id are inserted in the joint table.

user.activities = myActivities;
user.save();

Model looks like

User.hasMany( 
        'activities', // realtionship name on the model
        Activity, // the related model
        { 
            share: Number
        }, // join table extra fields
        {
            mergeId: 'user_id', // Custom name for column referencing this model
            mergeAssocId: 'activity_id', // Custom name for column referencing other model,
            reverse: 'users' // define the same relationship in the other model as well with this name (getUsers() from the other side)
        }
    );

The myActivities looks like

myActivities = [{ 'activity_id' : 1, 'activity_name' : 'like', 'share' : 1 }];

Any clue what's happening and how can I alter this? I prefer to have this so that I hit the database supposedly once for all of the myActivities elements instead of looping on each of the activities and using the setActivities function.