How do I use a main query value as a filter in a Subquery in cake php - sql

Question

How do I translate this into orm that uses the value form the main query in the sub query
Here is my sql subquery
(select ups.username from users as ups where ups.id=student_id) as StudentName
My main query gets the student_id from an inner join
Here is my Orm Subquery
->select([
'StudentName'=>'Users.username'])
->where(['Users.id'=>'students.studentid']);
When I run the query StudentName shows up blank, but if I manually set the value like this it returns a result
->select([
'StudentName'=>'Users.username'])
->where(['Users.id'=>'55']);

Answer 1

When using the key => value syntax, the right hand side value will always be subject to binding/escaping, unless it's an expression object. So your condition will bind Students.student_id as a literal string, or possibly as an integer, ie you end up with SQL like:
WHERE Users.id = 'Students.student_id'
or
WHERE Users.id = 0
Either pass a \Cake\Database\Expression\IdentifierExpression object, like:
->where(['Users.id' => $mainQuery->identifier('Students.student_id')])
or in older CakePHP versions:
->where(['Users.id' => new \Cake\Database\Expression\IdentifierExpression('Students.student_id')])
Or use the expression builder, which has support for comparing fields:
->where(function(\Cake\Database\Expression\QueryExpression $exp) {
return $exp->equalFields('Users.id', 'Students.student_id');
})
There's an example for your use case in the advanced conditions docs (see the exists() example).
See also
Cookbook > Database Access & ORM > Query Builder > Advanced Conditions