So, when calling insert the dbobject is already associated with _id, calling insert thus fails, because you already have a document with that _id in the collection, which should be unique (duplicate index error).. The unique property for an index causes mongodb to reject duplicate values for the indexed field. other than the unique constraint, unique indexes are functionally interchangeable with other mongodb indexes.. This index is created when the app is started for the first time. the rest of the times, mongoose checks to see if the index exists. if it doesn't it will create it..
This will keep the first unique document for each source_references.key value, and drop any subsequent documents that would otherwise cause a duplicate key violation. important notes : the dropdups option was removed in mongodb 3.0 , so a different approach will be required.. Inserting a duplicate value for any key that is part of a unique index, such as _id, throws an exception. the following attempts to insert a document with a _id value that already exists: copy. To index a field that holds an array value, mongodb creates an index key for each element in the array. these multikey indexes support efficient queries against array fields. multikey indexes can be constructed over arrays that hold both scalar values (e.g. strings, numbers) and nested documents..
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