How to Handle Nullable String Fields with GORM and sql.NullString?

Unable to Use Type String as sql.NullString

Query

In GORM, using sql.NullString for a field that may be NULL faces the error:

cannot use "a string goes here", (type string) as type sql.NullString in field value

while trying to execute a simple GORM validation example.

Resolution

sql.NullString is not a string type but a struct type defined as:

type NullString struct {
    String string
    Valid  bool // Valid is true if String is not NULL
}

To initialize it correctly, use the following syntax:

db.Create(&Day{
    Nameday:     "Monday",
    Dateday:     "23-10-2019",
    Something:   sql.NullString{String: "a string goes here", Valid: true},
    Holyday:     false,
})

Alternative Solution

Alternatively, to retain the simpler initialization syntax:

1. Define a custom nullable string type:

   type MyString string

2. Implement the sql.Scanner and driver.Valuer interfaces to handle value conversion.

3. Override the methods Value() and Scan() as follows:

   func (s MyString) Value() (driver.Value, error) {
       if s == MyStringNull {
           return nil, nil
       }
       return []byte(s), nil
   }
   
   func (s *MyString) Scan(src interface{}) error {
       switch v := src.(type) {
       case string:
           *s = MyString(v)
       case []byte:
           *s = MyString(v)
       case nil:
           *s = MyStringNull
       }
       return nil
   }

4. Declare the Something field as MyString and initialize it as intended:

   db.Create(&Day{
       Nameday:     "Monday",
       Dateday:     "23-10-2019",
       // Here, an untyped string constant will explicitly convert to MyString because they have the same underlying type.
       Something:   "a string goes here",
       Holyday:     false,
   })

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