How to Handle Nullable Strings in GORM Models?

Unable to Use String Type as sql.NullString

When creating a GORM model, you may encounter an error if you try to use a string type for a field that needs to be nullable. The issue arises when using sql.NullString for nullable fields.

Problem:

The error you're getting, "cannot use "a string goes here", (type string) as type sql.NullString in field value," indicates that you're trying to assign a string directly to a sql.NullString field. However, sql.NullString is not a string type, but a struct type designed to represent nullable strings.

Solution:

To resolve this issue, you need to initialize the sql.NullString field correctly. It should be initialized as:

<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 Solution:

If you prefer using a simplified syntax for nullable strings, you can create your own nullable string type by implementing the sql.Scanner and driver.Valuer interfaces, and leveraging the null byte to signal a NULL value.

<code class="go">type MyString string

const MyStringNull MyString = "\x00"

// implements driver.Valuer, will be invoked automatically when written to the db
func (s MyString) Value() (driver.Value, error) {
    if s == MyStringNull {
        return nil, nil
    }
    return []byte(s), nil
}

// implements sql.Scanner, will be invoked automatically when read from the db
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
}</code>

With this type, you can initialize the Something field as follows:

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

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