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Update create-a-unique-index.txt #1655

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9 changes: 9 additions & 0 deletions source/tutorial/create-a-unique-index.txt
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Expand Up @@ -38,6 +38,15 @@ The :ref:`_id index <index-type-id>` is a unique index. In some
situations you may consider using ``_id`` field itself for this kind
of data rather than using a unique index on another field.

The unicity constraint prevents two documents from the collection to
have the same value, but it does not prevent one specific document to
have a value duplicated. For instance, a unique index on ``a.b``
will let the following document exist:

.. code-block:: javascript

{ "a" : [ { "b": 1 }, { "b" : 1 } ] }

In many situations you will want to combine the ``unique`` constraint
with the ``sparse`` option. When MongoDB indexes a field, if a
document does not have a value for a field, the index entry for that
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