Add user authentication function

models/user.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"golang.org/x/crypto/bcrypt"
+	"google.golang.org/grpc/resolver/passthrough"
 )
 
 type User struct {
@@ -41,3 +42,24 @@ func (us *UserService) Create(email, password string) (*User, error) {
 	}
 	return &user, nil
 }
+
+func (us UserService) Authenticate(email, password string) (*User, error) {
+	user := User{
+		Email: strings.ToLower(email),
+	}
+
+	row := us.DB.QueryRow(`
+    SELECT id, password_hash
+    FROM users WHERE email=$1
+  `, email)
+	err := row.Scan(&user.ID, &user.PasswordHash)
+	if err != nil {
+		return nil, fmt.Errorf("authenticate: %w", err)
+	}
+
+	err = bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(password))
+	if err != nil {
+		return nil, fmt.Errorf("authenticate: %w", err)
+	}
+	return &user, nil
+}