Model exposes getById(id, [options]) method. Which accepts id string or instance of Key.
Note that in CouchbaseODM id and key represents different document values:

a key of a document is composed of static and dynamic (id) part.
Static part of key determines type of document within a bucket whereas dynamic part is actual identificator (id) usually represented by auto incremented integer or generated uuid string value.
Both static part of document's key and document's id forms a unique representation of the document within the bucket

Example

Document's key:

User_1 //auto-incremented integer

User_000a2644-a151-4771-98ef-132134e1606e //uuidv4 id

In both examples, static portion of document's key is represented by User_.
id value in first example is equal to 1 in second example id = 000a2644-a151-4771-98ef-132134e1606e

Getting a document by id/Key considering above document key samples would look like:

    return User.getById(1).then(function(user) {
        console.log(user);
    });

    //or

    return User.getById('000a2644-a151-4771-98ef-132134e1606e').then(function(user) {
        console.log(user);
    });

    //or

    const key = User.buildKey('000a2644-a151-4771-98ef-132134e1606e');
    return User.getById(key).then(function(user) {});

Model can only have one key generation strategy eg.: UUID4Key or IncrementalKey, not both

Static part of key can be customized by schemaSettings.key option on Model. For id generation there is couple of key generation strategies like UUID4Key or IncrementalKey .
if you have special needs, you can create a strategy yourself. See Customizing Document's Key