Why does the error "Unable to use type string as sql.NullString" occur in Go's GORM library, and how can it be resolved?

Understanding the "Unable to use type string as sql.NullString" Error

In Go, the gorm.Model provides functionality for working with databases using SQL. When creating database models using GORM, data types must be appropriately declared to match their corresponding SQL data types. The "Unable to use type string as sql.NullString" error arises when attempting to assign a string value to a field that has been declared as sql.NullString.

sql.NullString: A Struct, Not a String

The sql.NullString type is not a string type itself, but rather a struct. It comprises two fields: String (the actual string value) and Valid (a boolean indicating whether the String value is valid or NULL).

Initializing sql.NullString Properly

To correctly initialize a sql.NullString value, provide a sql.NullString struct with both the String and Valid fields set appropriately. For example:

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

Alternative: Defining a Nullable String Type

Alternatively, you can define a custom nullable string type, implementing the sql.Scanner and driver.Valuer interfaces. By leveraging the null byte, you can signal NULL values.

<code class="go">type MyString string

const MyStringNull MyString = "\x00"

// Implement driver.Valuer to convert to database value
func (s MyString) Value() (driver.Value, error) {
    if s == MyStringNull {
        return nil, nil
    }
    return []byte(s), nil
}

// Implement sql.Scanner to convert from database value
func (s *MyString) Scan(src interface{}) error {
    // Handle different types
    switch v := src.(type) {
    case string:
        *s = MyString(v)
    case []byte:
        *s = MyString(v)
    case nil:
        *s = MyStringNull
    }
    return nil
}</code>

With this approach, you can use your custom nullable string type as follows:

<code class="go">db.Create(&Day{
    Nameday:     "Monday",
    Dateday:     "23-10-2019",
    Something:   "a string goes here", // implicitly converted to MyString
    Holyday:      false,
})</code>

Note: Explicit conversion may be required when assigning typed string values to the custom nullable string type.

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