Are Objectids unique?
The ObjectID is automatically generated by the database drivers and will be assigned to the _id field of each document. ObjectID can be considered globally unique for all practical purposes. ObjectID encodes the timestamp of its creation time, which can be used for queries or to order by creation time.
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How does MongoDB maintain uniqueness?
To create a unique index, use the db. collection. createIndex() method with the single option set to true .
What is distinct mongoose?
MongoDB – Distinct() Method In MongoDB, the distinct() method finds the distinct values for a given field in a single collection and returns the results in an array. In this method, if the given field value is an array, this method will consider each value in the array as a separate value.
Why does MongoDB use _id?
In MongoDB, the _id field as the collection’s primary key so that each document can be uniquely identified in the collection. The _id field contains a unique ObjectID value.
How is MongoDB _id generated?
MongoDB uses ObjectIds as the default value for each document’s _id field, which is generated during the creation of any document. The complex combination of ObjectId makes all _id fields unique.
What does it mean to have a unique index in MongoDB?
Unique index and missing field¶. If a document does not have a value for the indexed field in a unique index, the index will store a null value for this document. Due to the unique restriction, MongoDB will only allow a document that lacks the indexed field.
How to enforce uniqueness on multiple fields in MongoDB?
Note that this methodology requires creating a unique ID for the primary_id field rather than allowing MongoDB to automatically create it when inserting the document. If you need to enforce uniqueness on multiple fields, each field will require its own proxy collection. The complete documentation of: createIndex() and shardCollection.
Why am I getting a duplicate key error in MongoDB?
Due to the unique restriction, MongoDB will only allow a document that lacks the indexed field. If there is more than one document without a value for the indexed field or the indexed field is missing, the index build will fail with a duplicate key error.