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node.jsmongodbmongoosediscriminator

Mongoose - how to find discriminators already in use

I'm using MongoDB and Mongoose in a REST API. Some deployments require a replica set, thus separate read/write databases, so as a result I have separate read/write connections in the API. However, more simple deployments don't need a replica-set, and in those cases I point my read/write connections to the same MongoDB instance and database.

My general approach is to create all models for both connections at API start up. Even when read/write conns are connecting to same database, I am able to create the same models on both connections without error.

let ReadUser = dbRead.model('User', userSchema);
let WriteUser = dbWrite.model('User', userSchema);
// no error even when dbRead and dbWrite point to same DB

Trouble comes when until I start using Mongoose Discriminators.

let ReadSpecialUser = ReadUser.discriminator('SpecialUser', specialUserSchema);
let WriteSpecialUser = WriteUser.discriminator('SpecialUser', specialUserSchema);

// Results in this Error when read and write point to same DB:
// Error: Discriminator with name "SpecialUser" already exists

I'm look for an elegant way to deal with this. Is there a way to query the db for discriminators that are already in use?

Solution

  • According to the Mongoose API docs the way to do this is to use Model.discriminators. So in the case above it would be

    ReadUser.discriminators

    or 

    WriteUser.discriminators

    However this doesn't return anything for me. What does work is using

    Object.keys(Model.discriminators)

    As expected this gets you an array of strings of the discriminator names you've set previously.

    If you want to use the existing discriminator model and know its name what you can do is use Model.discriminators.discriminatorName. In your example it would be:

    let ReadSpecialUserDocument = new ReadUser.discriminators.SpecialUser({
  key: value,
  key: value,
});

ReadSpecialUserDocument.save()

    This can be useful when you need to reuse the discriminator at different times, and its name is tied to your data in some way.