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Relationships that point to multiple records are displayed in the frontend in the “Related Data” and “Explorer” tabs.

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One-to-Many relationships

In a one-to-many relationship, one record from a Collection is attached to multiple records of another Collection. Think about countries and towns: a country has multiple towns, and each town belongs to a country. 
// Link 'countries' to 'towns'
agent.customizeCollection('countries', collection => {
  collection.addOneToManyRelation('myTowns', 'towns', {
    originKey: 'country_id',
    originKeyTarget: 'id', // Optional (uses primary key of countries by default)
  });
});

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Many-to-Many relationships

In a many-to-many relationship, 3 Collections are used instead of 2 to build the relationship. This allows multiple records from one Collection to be attached to multiple records from another Collection. For instance, on a movie recommendation website, each user can rate many movies, and each movie can be rated by many users. The 3 Collections used are users (the “origin”), ratings (the “through”), and movies (the “foreign” Collection). 
// Create one side of the relationship ...
agent.customizeCollection('users', collection => {
  collection.addManyToManyRelation('ratedMovies', 'movies', 'ratings', {
    originKey: 'user_id',
    foreignKey: 'movie_id',
  });
});

// ... and the other one
agent.customizeCollection('movies', collection => {
  collection.addManyToManyRelation('whoRatedThisMovie', 'users', 'ratings', {
    originKeyTarget: 'id', // Optional (uses primary key of movies by default)
    originKey: 'movie_id',
    foreignKey: 'user_id',
    foreignKeyTarget: 'id', // Optional (uses primary key of users by default)
  });
});

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External relationships

External relationships allow defining Collections which will only be available through the “Related Data” section of a given model.
External relationships do not support pagination.
const states = [
  { code: 'AK', name: 'Alaska', zip: [99501, 99950], closeTo: [] },
  { code: 'AL', name: 'Alabama', zip: [35004, 36925], closeTo: ['TE', 'MI', 'GE'] },
  { code: 'AR', name: 'Arkansas', zip: [71601, 72959], closeTo: ['OK', 'TX', 'LO'] },
  { code: 'AZ', name: 'Arizona', zip: [85001, 86556], closeTo: ['NM', 'CO', 'NE'] },
  { code: 'CA', name: 'California', zip: [90001, 96162], closeTo: ['OR', 'NE'] },
  // ....
];

agent.customizeCollection('address', collection => {
  collection.addExternalRelation('nearStates', {
    // Define schema of the records in the relationship.
    schema: { code: 'Number', name: 'String' },

    // Which fields are needed from the parent record to run the handler?
    // Dependencies are optional: by default only the primary key of address would be
    // provided.
    dependencies: ['country', 'zipCode'],

    // Compute list of records from the parent record
    listRecords: async ({ country, zipCode }) => {
      if (country === 'USA') {
        const state = states.find(s => s.zip[0] < zipCode && zipCode < s.zip[1]);
        return states.filter(s => state.closeTo.includes(s.code));
      }

      return [];
    },
  });
});