A Simple Login System

Schema is assumed to be:

// @upsert directive is important to detect conflicts.
email: string @index(exact) @upsert . # @index(hash) would also work
pass: password .

// Create a new transaction. The deferred call to Discard
// ensures that server-side resources are cleaned up.
txn := client.NewTxn()
defer txn.Discard(ctx)

// Create and execute a query to looks up an email and checks if the password
// matches.
q := fmt.Sprintf(`
    {
        login_attempt(func: eq(email, %q)) {
            checkpwd(pass, %q)
        }
    }
`, email, pass)
resp, err := txn.Query(ctx, q)

// Unmarshal the response into a struct. It will be empty if the email couldn't
// be found. Otherwise it will contain a bool to indicate if the password matched.
var login struct {
    Account []struct {
        CheckPwd bool `json:"checkpwd(pass)"`
    } `json:"login_attempt"`
}
err = json.Unmarshal(resp.GetJson(), &login); err != nil {

// Now perform the upsert logic.
if len(login.Account) == 0 {
    fmt.Println("Account doesn't exist! Creating new account.")
    mu := &protos.Mutation{
        SetJson: []byte(fmt.Sprintf(`{ "email": %q, "pass": %q }`, email, pass)),
    }
    _, err = txn.Mutate(ctx, mu)
    // Commit the mutation, making it visible outside of the transaction.
    err = txn.Commit(ctx)
} else if login.Account[0].CheckPwd {
    fmt.Println("Login successful!")
} else {
    fmt.Println("Wrong email or password.")
}