Why Does Type Assertion Fail When Implementing `Valuer` and `Scanner` Interfaces in Go?

Understanding Golang Type Assertions

In Go, one can define custom types based on existing types using type aliases. In the given scenario, the Role type is an alias for the built-in string type. However, upon implementing the Valuer and Scanner interfaces for this custom type, an error is encountered.

Addressing the Error

The error message suggests that there is an issue with the type conversion in the Scan method. Specifically, it complains about the attempt to assign the converted value to a pointer to a Role.

The first function, Scan, is responsible for extracting a value from the database and converting it to the appropriate type. The issue lies in the assignment line:

r = (*Role)(value.(string))

The expression value.(string) attempts to convert the value parameter to a string. However, the assignment tries to then assign this converted value to a pointer to a Role. This is incorrect, as the destination should be of type *Role, not Role. The corrected code is:

func (r *Role) Scan(value interface{}) error {
    *r = Role(value.(string))
    return nil
}

The second function, Value, is responsible for converting a value to a database-compatible format. The given code is incorrect because it is not following the expected function signature for driver.Valuer. The corrected code is:

func (r Role) Value() (driver.Value, error) {
    return string(r), nil
}

Conclusion

By correcting the type conversion in the Scan method and adjusting the function signature in the Value method, the code should work as intended, allowing the custom Role type to interact with the database driver.

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