Optional fields

A Toql query can select individual fields from a struct. However fields must be Option for this, otherwise they will always be selected in the SQL statement, regardless of the query.

Example:

 

# #![allow(unused_variables)]
#fn main() {
  #[derive(Toql)]
    struct User {

        #[toql(key)]
        id: u32,			// Always selected in SQL (keys must not be optional)

        age: u8,			// Always selected in SQL

        firstname: Option<String>	// Selectable field of non nullable column
        middlename: Option<Option<String>>// Selectable field of nullable column

        #[toql(preselect)]	
        lastname: Option<String>	// Always selected in SQL, nullable column
  }
#}

You noticed it: Nullable columns that should always be selected must be annotated with preselect.

Preselection and joins

Preselected fields on joined structs are selected, if

  • A join itself is preselected
  • or at least one field on that join is selected

Preselection example

 

# #![allow(unused_variables)]
#fn main() {
  #[derive(Toql)]
    struct User {

        #[toql(key)]
        id: u32,

        #[toql(join())]
        native_language: Language,	// Preselected inner join

        #[toql(join())]
        foreign_language: Option<Option<Language>>,
    }

    #[derive(Toql)]
    struct Language {
            #[toql(key)]
            id: u32,

            code: Option<String>
    }
#}

Above id in User is always selected, because it's not Option. As native_language is a preselected (inner) join, its id will also always be selected. But on the contrary foreign_language is a selectable (left) join. id will only be selected if the query requests any other field from that join. For example with foreignLanguage_code.

Preselection on parent paths

One more thing: If a field on a related struct is selected, all preselected fields from the path line will be selected too.

Lets assume we have a user that has an address, which contains country information.

The query

address_country_code

would therefore

  • select code from the table Country
  • select all preseleted fields from table Country
  • select all preseleted fields from table Address
  • select all preseleted fields from table User