What is the proper way to serialize nested Flask-SQLAlchemy objects using Flask-Marshmallow?

Question

ORM

Good afternoon.
There are two tables:

Organizations

class Organization(mixins.PaginatedAPIMixin, db.Model):
    __tablename__ = 'organization'

    id = db.Column(db.Integer, primary_key=True, nullable=False, unique=True)
    name = db.Column(db.String(128), nullable=False)
    document_cipher = db.Column(db.String(6), unique=True)
    address = db.Column(db.String(256))
    website = db.Column(db.String(32))
    is_active = db.Column(db.Boolean, default=True)
    organization_type_id = db.Column(db.ForeignKey('organization_type.id'),
                                     nullable=False, index=True)
    organization_type = db.relationship('OrganizationType', backref='organizations', lazy=True)

and

Organization types

class OrganizationType(mixins.PaginatedAPIMixin, db.Model):
    __tablename__ = 'organization_type'

    id = db.Column(db.Integer, primary_key=True, unique=True)
    type = db.Column(db.String(45), nullable=False, unique=True)
    description = db.Column(db.Text)

Also, there are schemes for their (de)serialization:

Organization Chart

class OrganizationsSchema(ma.ModelSchema):
    class Meta:
        model = Organization

and

Organization Type Schema

class OrganizationsTypesSchema(ma.ModelSchema):
    class Meta:
        model = OrganizationType

In the end, I want to get this:

serialized object

{
  "_links": {},
  "_meta": {},
  "items": [
    {
      "address": "address",
      "document_cipher": "ABCD",
      "id": 1,
      "name": "ООО \"Horns'n'Hoofs\"",
      "organization_type": {
            "id": 1,
            "name": "Owner",
            "description": "spam"
            },
      "website": "http://hnh.ololo"
    }
  ]
}

(I put an array in organization_type to unify the output, there will be many sub-objects in some places), but at the moment, with the code above, I have just a property ID in organization_type ... How to deal with this?