JPA in clause using Specification - JHipster approach

I have 2 Entities A, B. A contains a Set<B> and I need in the QueryService to filter by a field X of B.

I see Predicate, but I do not understand how to use it.

    	StringFilter lf = new StringFilter(); lf.setEquals(value);
    	return (root, query, builder) -> {
    		Predicate p = builder.in(expression);
    		return builder.and(p);
    	};

What put in "expression"???? to obtain something like:

SELECT * FROM A WHERE 
A.id in (select b.id_A from b where b.code_field ='alfacentauri')

Thanks!

You should use Subquery inside predicate this way

return (root, query, builder) -> {
    Subquery<Long> subquery = query.subquery(Long.class); // if b.id_A has Long type
    Root<B> subRoot = subquery.from(B.class);
        
    final Predicate codeFieldPredicate = 
        builder.equal(subRoot.get("code_field"), "alfacentauri");
        
    subquery.select(subRoot.get("id_A")).where(codeFieldPredicate);

    return builder.in(root.get("id")).value(subquery);
}

Another approach is to use exists predicate

return (root, query, builder) -> {
    Subquery<Long> subquery = query.subquery(Long.class);
    Root<B> subRoot = subquery.from(B.class);
            
    final Predicate mainQueryPredicate = 
        builder.equal(root.get("id"), subRoot.get("id_A"));
    final Predicate codeFieldPredicate = 
        builder.equal(subRoot.get("code_field"), "alfacentauri");
            
    subquery.select(subRoot.get("id_A")).where(mainQueryPredicate , codeFieldPredicate);
    
    return builder.exists(subquery);
}

It generates a bit different query, but result will be as expected