[Bug] The filter operator "_child" is missing

[To-om]

Request Type

Bug

Work Environment

TheHive 4.0.0-RC2

Problem Description

In TheHive 3, the filter operators "_parent" and "_child" are used to build complex filter over object hierarchy (filter on case and related tasks for example).
In TheHive 4, the operator "_parent" exists but "_child" is missing. The latter is not used by the frontend but by TheHive4py.
The following Python code

query = And(
    Eq("title", "[DATALEAK] test"),
    Child('case_artifact', And(
        Eq('dataType', 'mail'),
        Eq('data', 'my@email.com')
    ))
)

which produces the raw query

{
  '_and': [{
    '_field': 'title',
    '_value': '[DATALEAK] test'
  }, {
    '_child': {
      '_type': 'case_artifact',
      '_query': {
        '_and': [{
          '_field': 'dataType',
          '_value': 'mail'
        }, {
          '_field': 'data',
          '_value': 'my@email.com'
        }]
      }
    }
  }]
}

doesn't work.