Just as insert now always creates an _id field, update should do the same to be consistent. See SERVER-12331 as guide.
Currently update only creates/requires an _id field if there is an _id index. It seems we should now ensure an the _id field for each updated document modified/inserted.
- is duplicated by
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SERVER-12523 When creating collection with autoIndexId set to false, upserts create documents with no _id
- Closed
- related to
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SERVER-8237 autoIndexId:false should be disabled
- Closed
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SERVER-12331 Insert creates the _id field, always
- Closed