Assossiate UID Authentication with key user Database

When I create a user, I use the method Auth to create email and password in authentication and the same time call another service then I save this user in Database
my authprovider.ts

  createUser(user: any, path: any) {
    return this.afu.auth.createUserWithEmailAndPassword(, user.password)
                                .then((res) => {
                                  return, path)

this service save send to another provider that to save in Database this user created

  save(user: any, path: string) {
    return new Promise((resolve) => {
      this.db.list(this.PATH + path)
              .then(() => resolve())

Now, how to assossiate UID of authentication with Database