One case that could break by this change is if you relied on insert's old default of being a no-op if an object with the same _id already exists. This can be us

One case that could break by this change is if you relied on insert's old default of being a no-op if an object with the same _id already exists. This can be used to efficiently insert a default document without race conditions. Example at https://github.com/RedBeard0531/Mongurl/blob/master/mongurl.py#L51-57 https://github.com/RedBeard0531/Mongurl/blob/master/mongurl.... That said, it makes sense for this to be opt-in behavior rather than the default.