Is it possible to create a shard key from an array in mongodb?

Question

I am trying to create shard key from an array but it is showing me error -

errmsg" : "can't shard collection 'inno.inno_pub' with unique index on { mid: 1.0 } and proposed shard key { mid: 1.0, t.category: 1.0, t.value: 1.0 }. "Uniqueness can't be maintained unless shard key is a prefix"

Answer 1

As per release 4.4
Refinable shard keys are introduced, which simply means you can keep rebalancing with a more fine-grained level even after you've defined the shard ley by adding/suffix/appending the key for better distribution/cardinality as per the data growth/added new shards in cluster

Copied from the official source:

MongoDB 4.4 brings the ability to refine the shard key itself, and to create compound hashed shard keys to allow for an even more fine-grained distribution of data within the cluster.

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Shard keys cannot be arrays. sh.shardCollection() will fail if any key has an array value and inserting an array into that field is not allowed.

Holds good with version 4.2

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But, as an important note with the release of new version starting 4.2, the following statement does not apply.

"Once inserted, a document's shard key value cannot be modified" .

So the answer to the question, Can shard key be changed?

Although you cannot select a different shard key for a sharded collection, starting in MongoDB 4.2, you can update a document's shard key value unless the shard key field is the immutable _id field

Answer 2

From Kristina Chodorow's book MongoDB - The Definitive Guide:

Shard keys cannot be arrays. sh.shardCollection() will fail if any key has an array value and inserting an array into that field is not allowed. Once inserted, a document's shard key value cannot be modified. To change a document's shard key, you must remove the document, change the key, and reinsert it. Thus, you should choose a field that is unchangeable or changes frequently.