Mongoose error: nesting Schemas

mongodb, mongoose, node.js

Solution

MongoDB is not a relational database. This can cause confusion for some who are used to the RDBS model (I still get tripped up occasionally...but I'm really not a DB guy).

Oftentimes, you'll find it beneficial to reference other documents in your Mongo entities. Mongoose schemas provide a very simple and effective way to do this that feels very relational.

When defining a schema that will store a reference to a different type of document, you define the relevant property as an object with a `type` and a `ref`. Typically when defining schema properties, you can simply say: `a: Number`; however, Mongoose provides many different options for a schema property other than type:

a: {
   type: Number,
   required: true   
}

Setting `required: true` will prevent us from saving a document where the `a` property is not present.

Once you understand how to define your schemas with object definitions, you can leverage Mongoose's population mechanic:

a: {
   type: Mongoose.Schema.ObjectId,
   ref: 'a'
}

This tells Mongoose to store the `ObjectId` (a Mongoose-specific identifier) of a specific `a` document as the `a` property of our schema. Still following me?

When setting this property on a Mongoose document, you can simply say: `doc.a = myA`. When you go to save `doc`, Mongoose will automagically make the conversion and only store the ID in your database.

When retrieving a document that references another schema, you'll need to populate. I won't go into that, but its pretty simple - check out the documentation.

Problem

I have a question about nesting Mongoose schema. Here is a simple code snippet ``` var aSchema = new Schema({bar: String}); var bSchema = new Schema({a: aSchema, foo: String}); var cSchema = new Schema({as: [aSchema], foo:String}); ``` This will throw `TypeError` on `bSchema`: `TypeError: Undefined type at 's' Did you try nesting Schemas? You can only nest using refs or arrays.`, but works fine for `cSchema`. Just want to ask why `bSchema` does not work. Cannot find explanation in Mongoose doc. Thanks.

Original source