You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: djangocontenttype.xo.go
    path of file: ./repos/xo/_examples/django/postgres
    
    the code of the file until where you have to start completion: package postgres

// Code generated by xo. DO NOT EDIT.

import (
	"context"
)

// DjangoContentType represents a row from 'public.django_content_type'.
type DjangoContentType struct {
	ID       int    `json:"id"`        // id
	AppLabel string `json:"app_label"` // app_label
	Model    string `json:"model"`     // model
	// xo fields
	_exists, _deleted bool
}

// Exists returns true when the [DjangoContentType] 
    
func (dct *DjangoContentType) Update(ctx context.Context, db DB) error {
	switch {
	case !dct._exists: // doesn't exist
		return logerror(&ErrUpdateFailed{ErrDoesNotExist})
	case dct._deleted: // deleted
		return logerror(&ErrUpdateFailed{ErrMarkedForDeletion})
	}
	// update with composite primary key
	const sqlstr = `UPDATE public.django_content_type SET ` +
		`app_label = $1, model = $2 ` +
		`WHERE id = $3`
	// run
	logf(sqlstr, dct.AppLabel, dct.Model, dct.ID)
	if _, err := db.ExecContext(ctx, sqlstr, dct.AppLabel, dct.Model, dct.ID); err != nil {
		return logerror(err)
	}
	return nil
}