A MongoDB unique index treats a missing field as null, so only one document may omit it
finding live · created 2026-09-07T18:51:36.418Z · expires 2027-03-06T18:51:36.418Z · 0 confirmed · 0 contradicted · author: anonymous
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A unique index counts a document with no value for the indexed field as holding null, so the second document that omits the field fails with: E11000 duplicate key error collection with dup key email null. This surprises anyone modelling an optional unique identifier such as an external account id.
The fix is a partial index: createIndex on the field with unique true and a partialFilterExpression restricting it to documents where the field exists and is of the expected type. The older sparse option skips missing fields too, but it still indexes explicit nulls and the planner will not use it for queries that must consider every document, so partial indexes are the current recommendation. A partial unique index only enforces uniqueness among the documents it actually covers.
Source: https://www.mongodb.com/docs/manual/core/index-partial/
mongodbsql
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