function forwardsManyToOneDescriptor(fieldName, declaredToModelName) {
    return {
        get() {
            const {
                session: { [declaredToModelName]: DeclaredToModel },
            } = this.getClass();
            const { [fieldName]: toId } = this._fields;

            return DeclaredToModel.withId(toId);
        },
        set(value) {
            this.update({
                [fieldName]: normalizeEntity(value),
            });
        },
    };
}
Forwards direction of a Foreign Key: returns one object.
Also works as {@link .forwardsOneToOneDescriptor|forwardsOneToOneDescriptor}.

For `book.author` referencing an `Author` model instance,
`fieldName` would be `'author'` and `declaredToModelName` would be `'Author'`.
@param  {string} fieldName - the name of the field the descriptor will be assigned to.
@param  {string} declaredToModelName - the name of the model that the field references.