How to define ManyToOne and map to custom data from server?

When you're talking about associations it's better to use concrete examples because of the way the name generation works.

You can do this:

Code:

Ext.define('User', {
    extend: 'Ext.data.Model',
    fields: ['name']
});

Ext.define('Post', {
    extend: 'Ext.data.Model',
    fields: ['content', {
        name: 'userId',
        reference: {
            type: 'User',
            inverse: 'comments'
        }
    }]
});

Ext.onReady(function () {

    User.load(17, {
        callback: function(rec) {
            var comments = rec.comments();
            console.log(comments.first().get('content'));
        }
    });

});

Server returns:

Code:

{
    "id": 17,
    "name": "Foo Bar",
    "comments": [{
        "id": 1,
        "userId": 17,
        "content": "Foo"
    }]
}