How to solve error "Operator 'getitem' is not supported on this expression" when using case()

Question

I'm trying to convert the following SQL into SQLAlchemy:

select t1.id, t1.field_A,
    max(case when t2.field_B = 1 then t2.field_C end) test_2_field_b_1,
    max(case when t2.field_B = 2 then t2.field_C end) test_2_field_b_2
from test_1 t1
inner join test_2 t2 on t2.field_A = t1.field_A
group by t1.id, t1.field_A

I've got as far as:

qry = session.query(
    Test1.id_,
    Test2.field_A,
    func.max(case((Test2.field_B.__eq__(1), "Test2.field_C"))).label("test_2_field_b_1"),
    func.max(case((Test2.field_B.__eq__(2), "Test2.field_C"))).label("test_2_field_b_2"),
)
qry = qry.select_from(Test1)
qry = qry.join(Test2, Test2.field_A.__eq__(Test1.field_A))
qry = qry.group_by(Test1.id_, Test2.field_A)

But I'm getting the following error:

NotImplementedError: Operator 'getitem' is not supported on this expression

On the line:

func.max(case((Test2.field_B.__eq__(1), "Test2.field_C"))).label("test_2_field_b_1"),

SO won't let me post the whole traceback as it says there is too much code!

Where am I going wrong?

Answer 1

case takes a list of when criteria as its first argument, so you need to bracket accordingly:

sa.case([(Model.column == value)])

So your code should look like this:

qry = session.query(
    Test1.id_,
    Test2.field_A,
    sa.func.max(sa.case([(Test2.field_B == 1, "Test2.field_C")])).label("test_2_field_b_1"),
    sa.func.max(sa.case([(Test2.field_B == 2, "Test2.field_C")])).label("test_2_field_b_2"),
)
qry = qry.select_from(Test1)
qry = qry.join(Test2, Test2.field_A == Test1.field_A)