如何在MongoDB中指定_id和对象ID数组作为复合主键？

Question

In the MongoDB database, a Collection was created, and inserted with some data:

>>> db.createCollection("Restaurants")
>> { "ok" : 1 }

>> db.Restaurants.insert({ "_id": "coolDinings",
>> "AppliedParameters": [ ObjectId("5617a7a52da21a733547b8da"),
>> ObjectId("5617a5102da21a733547b8d9") ], "Review": "excellent    SeaFood", "Region": "USA" })

MongoDB default behaviour is to consider "_id" as the primary key.

However, I wanted a composite primary key consisting of the following fields:

• -1) "_id"
• -2) "AppliedParameters"

"AppliedParameters" is an array of ObjectIds that can really vary in terms of number of ObjectIds it stores which can range from 0 to around 20 or 30 ObjectIds.

I know the following code will create an index over both the aforementioned fields:

db.Restaurants.ensureIndex( { "_id": 1,"AppliedParameters": 1 }, { unique:true, sparse:true } ); 

However, my desire was to ensure that the aforementioned line of code will create a composite primary key.

In any case I ran the code.

In order to check if the composite key was created, I just reran the very first data insert again because I was hope it would complain by throwing an error related to composite primary key duplicates:

    >> db.Restaurants.insert({ "_id": "coolDinings",
    >> "AppliedParameters": [ ObjectId("5617a7a52da21a733547b8da"),
    >> ObjectId("5617a5102da21a733547b8d9") ], "Review": "excellent        SeaFood", "Region": "USA" })

However, it only gave the following error:

E11000 duplicate key error index: test.Restaurants.$ id dup key: { : "coolDinings" }

The aforementioned error seems to indicate that the "_id" is duplicated. I want the Mongo client to state something along the lines of duplicate composite primary keys are Not allowed.

How do I go about the said task?

Answer 1

There's no "composite" primary keys in mongodb (in a sense that they span multiple fields). _id is the one and only primary key.

If you want a complex primary key, composed of several pieces of data, insert them all into the _id field.

Note that _id value can't be an array (must be one of primitives or an object).

Can't do:

{ _id: [1, 2, 3] }

Can do:

{ _id: { a: 1, b: [2, 3] } }