Fix Dataservice.datasets sometimes non existent instead of being empty list #3242
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maudetes
reviewed
Jan 14, 2025
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Thank you for the test and the explanation 🙏
Seeing as it can happen on any ListField, I think it will be difficult to maintain to add this custom logic to all the post_save places?
Maybe we should be resilient with the ListField being unset? We're currently not resilient on resource upload and dataservice filtering on no datasets.
| # See https://github.com/MongoEngine/mongoengine/issues/267#issuecomment-283065318 | ||
| # Setting it explicitely to an empty list should NOT remove the field. | ||
| dataservice.datasets = [] | ||
| dataservice.save() |
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Suggested change
| dataservice.save() | |
| dataservice.save() | |
| dataservice.reload() |
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|
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| # Setting it explicitely to a non-empty list should work as expected | ||
| dataservice.datasets = [dataset] | ||
| dataservice.save() |
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| dataservice.save() | |
| dataservice.save() | |
| dataservice.reload() |
…ing empty list" This reverts commit fbe7518.
…issing ListFields
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Fixes #1430
The main issue seems to be because of a bug in mongoengine.
This first draft is a hackish fix: when a
ListFieldis updated, if it's changed to be the empty list (or if the result is an empty list, if you dropped the last element for example), instead of being set to[], it's in fact unset (the field is removed from the document).The added test showcases this.
This happens in BaseDocument._delta: if the value isn't different from the default value (which is
[]by default on aListField), then the field is unset (instead of set to be empty).The hackish fix updates the
datasetsfield to be[]in thepost_savesignal. This means that we might want to do the same thing forDataset.resources, and maybe for otherListFields just in case.We might want to follow another route, maybe subclassing
MongoEngine.BaseDocumentorMongoEngine.Documentto overwrite the_deltamethod, or theMongoEngine.Document._get_update_docmethod to not unset a field if it's aListField.For the record, there's an existing opened PR on mongoengine (that I didn't review, so no clue if it fixes it properly), but it's been open for 3 years already, so not sure if or when it'll be merged.