How to Initialize a sql.NullString Field in GORM?

Unable to Use Type String as SQL.NullString

Problem

When creating a GORM model with a field of type sql.NullString, an attempt to initialize the field with a string value results in the error:

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

Solution

The sql.NullString type is not actually a string type but a struct that encapsulates a string and a boolean flag indicating whether the string is valid (not NULL). To initialize a sql.NullString field, it must be initialized with a struct value, not a string value.

The following code demonstrates how to correctly initialize a sql.NullString field:

<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

Alternatively, a custom nullable string type can be defined that implements the sql.Scanner and driver.Valuer interfaces and uses a null byte to signal a NULL value. With this custom type, the original syntax can be used to initialize a nullable string field.

Custom Type:

<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>

Usage:

<code class="go">db.Create(&Day{
  Nameday:     "Monday",
  Dateday:     "23-10-2019",
  // no need for explicit typing/conversion
  Something:   "a string goes here",
  Holyday:     false,
})</code>

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