Adding concurrency control in our updateMovieHandler and documenting it.

internal/data/models.go

@@ -6,8 +6,10 @@ import (
 )
 
 // ErrRecordNotFound : Define a custom ErrRecordNotFound error. We'll return this from our Get() method when looking up a movie that doesn't exist in our database.
+// ErrEditConflict : This error is used when a data race occurs.
 var (
 	ErrRecordNotFound = errors.New("record not found")
+	ErrEditConflict   = errors.New("edit conflict")
 )
 
 // Models : Wraps the MovieModel. We'll add other models to this, like a UserModel and PermissionModel, as our build progresses.