Custom data source and filtering by linked fields

Hello,
I’m trying to use doo.extensions.customDataSource.getData with a filter on a relation column.
The column is called Servis_generator - Číslo servisního protokolu (it’s a relation to another table).

In the console I see the column like this (without filter):

My test script looks like this:

(async (doo: IDoo) => { // do not change this line
// place your code here

const options = {
limit: 100,
filter: ‘Servis_generator - Číslo servisního protokolu(eq)117’,
};

const result = await doo.extensions.customDataSource
.getData('Položky k fakturaci',options);

console.log(result);

}) // do not change this line

…and the result of this operation is unfiltered data, as if no filter had been applied.

I’m not sure whether the problems are caused by incorrect syntax, diacritics, or the fact that the field is a reference to another table and is not included in every record.

Thanks for the advice!

Regards,
Zdenek