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As part of my application requirements, I have a limit of 30 characters for an ID field. This is out of my control and I am wondering if the MongoDB default _id fields will work for me. It appears as though the default _id field is 24 characters long. That works for me, but I am wondering if this is likely to change in the future. I am well aware that things can always change, but, for the next year or two, can I expect there to be 24 character default _id fields?
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Properties of the Object IDAn object ID is 24 characters long with two characters taking up to one byte, thus containing a total of 12 bytes.
The _id Field In MongoDB, each document stored in a collection requires a unique _id field that acts as a primary key. If an inserted document omits the _id field, the MongoDB driver automatically generates an ObjectId for the _id field.
Architecturally, by default the _id field is an ObjectID, one of MongoDB's BSON types. The ObjectID is the primary key for the stored document and is automatically generated when creating a new document in a collection.
According to MongoDB's manual the answer is yes, it's unique by default: MongoDB creates the _id index, which is an ascending unique index on the _id field, for all collections when the collection is created. You cannot remove the index on the _id field.
They aren't actually 24 characters - they are 12 bytes (24 characters in hex representation). And yes, that will be the case for the foreseeable future.
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