Database relationships

MongoDB is schema-less (no relations)
Mongoose provides a schema (and relations)

Database relationships

1 to 1
1 to n (One to Many)
n to m (Many to Many)

One to One

One item at one side has a single matching item at the other, and viceversa
Implemented with a reference in the main one (alternatively, a reference at each end)

const countrySchema = new mongoose.Schema({
  name: String,
  capital: {
    type: ObjectID,
    ref: ''
  }
});

const Country = mongoose.model('Country', countrySchema);

const capitalCitySchema = new mongoose.Schema({
  name: String
});

const CapitalCity = mongoose.model('CapitalCity', capitalCitySchema);

const countrySchema
= new mongoose.Schema({
name: String,
capital: {
type: ObjectID,
ref: ''
}
});

const Country =
mongoose.model('Country', countrySchema);

const capitalCitySchema
= new mongoose.Schema({
name: String
});

const CapitalCity =
mongoose.model('CapitalCity',
capitalCitySchema);

const country = new Country({ name: 'Spain' });

const capitalCity = new CapitalCity({ name: 'Madrid' });

console.log(country.capital) // Madrid

console.log(capitalCity.country) // Spain

One to Many

One item at side A has several matching ones at side B
One item at side B has a single matching item at side A
Implemented with a reference to the parent, or a list of references to the childrens (alternatively, with a junction table)

Many to Many

One item at side A has several matching ones at side B
One item at side B has several matching items at side A
Usually implemented with a junction table (alternatively, a list of references at each end)

+ info

https://en.wikipedia.org/wiki/One-to-one_(data_model)
https://en.wikipedia.org/wiki/One-to-many_(data_model)
https://en.wikipedia.org/wiki/Many-to-many_(data_model)