Document how parent context fields are included in the child record

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/sdk/-/issues/303

Originally created by @edgarrmondragon on 2022-01-07 17:23:58

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When user parent-child streams with the SDK as documented in https://sdk.meltano.com/en/latest/parent_streams.html

class GitlabStream(RESTStream):
    # Base stream definition with auth and pagination logic 
    # This logic works for other base classes as well, including Stream, GraphQLStream, etc.


class EpicsStream(GitlabStream):

    name = "epics"

    # ...

    def get_child_context(self, record: dict, context: Optional[dict]) -> dict:
        """Return a context dictionary for child streams."""
        return {
            "group_id": record["group_id"],
            "epic_id": record["id"],
            "epic_iid": record["iid"],
        }


class EpicIssuesStream(GitlabStream):
    # Note that this class inherits from the GitlabStream base class, and not from 
    # the EpicsStream class.

    name = "epic_issues"

    # EpicIssues streams should be invoked once per parent epic:
    parent_stream_type = EpicsStream  

    # Assume epics don't have `updated_at` incremented when issues are changed:
    ignore_parent_replication_keys = True

    # Path is auto-populated using parent context keys:
    path = "/groups/{group_id}/epics/{epic_iid}/issues"

    # ...

the docs fail to mention that group_id, epic_id and epic_iid are all added to the child epic_issues records and should the dev want them, they need to be included in the stream schema.

[MeltyBot]

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