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Projection type safety

Applies to ✅ Open Source Edition   ✅ Express Edition   ✅ Professional Edition   ✅ Enterprise Edition

Since jOOQ 3.0, records and row value expressions up to degree 22 are now generically typesafe. This is reflected by an overloaded SELECT (and SELECT DISTINCT) API in both DSL and DSLContext. An extract from the DSL type:

// Non-typesafe select methods:
public static SelectSelectStep<Record> select(Collection<? extends SelectField<?>> fields);
public static SelectSelectStep<Record> select(SelectField<?>... fields);

// Typesafe select methods:
public static <T1>         SelectSelectStep<Record1<T1>>         select(SelectField<T1> field1);
public static <T1, T2>     SelectSelectStep<Record2<T1, T2>>     select(SelectField<T1> field1, SelectField<T2> field2);
// [...]

The type that is being projected is the org.jooq.SelectField, see also the next section about SelectField. Since the generic R type is bound to some Record[N], the associated T type information can be used in various other contexts, e.g. the IN predicate. Such a SELECT statement can be assigned typesafely:

Select<Record2<Integer, String>> s1 = create.select(BOOK.ID, BOOK.TITLE);
Select<Record2<Integer, String>> s2 = create.select(BOOK.ID, trim(BOOK.TITLE));

// Alternatively, just use var to infer the type:
var s3 = create.select(BOOK.ID, trim(BOOK.TITLE));

For more information about typesafe record types with degree up to 22, see the manual's section about Record1 to Record22.

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