Population

There are no joins in RxDB but sometimes we still want references to documents in other collections. This is where population comes in. You can specify a relation from one RxDocument to another RxDocument in the same or another RxCollection of the same database. Then you can get the referenced document with the population-getter.

This works exactly like population with mongoose.

Schema with ref

The ref-keyword in properties describes to which collection the field-value belongs to (has a relationship).

export const refHuman = {
    title: 'human related to other human',
    version: 0,
    properties: {
        name: {
            primary: true,
            type: 'string'
        },
        bestFriend: {
            ref: 'human',     // refers to collection human
            type: 'string'    // ref-values must always be string or ['string','null'] (primary of foreign RxDocument) 
        }
    }
};

You can also have a one-to-many reference by using a string-array.

export const schemaWithOneToManyReference = {
  version: 0,
  type: 'object',
  properties: {
    name: {
      type: 'string',
      primary: true
    },
    friends: {
      type: 'array',
      ref: 'human',
      items: {
        type: 'string'
      }
    }
  }
};

populate()

via method

To get the referred RxDocument, you can use the populate()-method. It takes the field-path as attribute and returns a Promise which resolves to the foreign document or null if not found.

await humansCollection.insert({
  name: 'Alice',
  bestFriend: 'Carol'
});
await humansCollection.insert({
  name: 'Bob',
  bestFriend: 'Alice'
});
const doc = await humansCollection.findOne('Bob').exec();
const bestFriend = await doc.populate('bestFriend');
console.dir(bestFriend); //> RxDocument[Alice]

via getter

You can also get the populated RxDocument with the direct getter. Therefore you have to add an underscore suffix _ to the fieldname. This works also on nested values.

await humansCollection.insert({
  name: 'Alice',
  bestFriend: 'Carol'
});
await humansCollection.insert({
  name: 'Bob',
  bestFriend: 'Alice'
});
const doc = await humansCollection.findOne('Bob').exec();
const bestFriend = await doc.bestFriend_; // notice the underscore_
console.dir(bestFriend); //> RxDocument[Alice]

Example with nested reference

const myCollection = await myDatabase.addCollections({
  human: {
    schema: {
      version: 0,
      type: 'object',
      properties: {
        name: {
          type: 'string'
        },
        family: {
          type: 'object',
          properties: {
            mother: {
              type: 'string',
              ref: 'human'
            }
          }
        }
      }
    }
  }
});

const mother = await myDocument.family.mother_;
console.dir(mother); //> RxDocument

Example with array

const myCollection = await myDatabase.addCollections({
  human: {
    schema: {
      version: 0,
      type: 'object',
      properties: {
        name: {
          type: 'string'
        },
        friends: {
          type: 'array',
          ref: 'human',
          items: {
              type: 'string'
          }
        }
      }
    }
  } 
});

//[insert other humans here]

await myCollection.insert({
  name: 'Alice',
  friends: [
    'Bob',
    'Carol',
    'Dave'
  ]
});

const doc = await humansCollection.findOne('Alice').exec();
const friends = await myDocument.friends_;
console.dir(friends); //> Array.<RxDocument>

If you are new to RxDB, you should continue here