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Relationships that point to a single record are displayed in your back-office as links. Once configured, they appear in charts, filters, scopes, and segments.
For performance reasons when sorting a Table View on customizer-defined relationships, Forest will always sort on the primary key of the related collection.

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Many-to-One relationships

Many-to-One relationships are by far the most common type of relationship: many records from a Collection are attached to another Collection record. Think about countries and towns: a town belongs to a single country, but each country can have multiple towns. 
agent.customizeCollection('towns', collection =>
  collection.addManyToOneRelation('country', 'countries', {
    foreignKey: 'country_id',
    foreignKeyTarget: 'id', // Optional (uses `country` primary key by default)
  }),
);

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One-to-One relationships

In a one-to-one relationship, there is a one-to-one mapping between records in 2 Collections. The relationship can be unset for some records, but no record from the first Collection can be linked to more than one record in the other Collection. Think about cities and mayors: A city can have at most one mayor, and each mayor belongs to a single city.
Take note that the inverse of a one-to-one is a many-to-one.This may seem counter-intuitive: the side of the relationship which should be configured as many-to-one is the one that carries the foreign key.
// Configure one side of the relationship ...
agent.customizeCollection('mayors', collection => {
  collection.addOneToOneRelation('city', 'cities', {
    originKey: 'mayor_id',
    originKeyTarget: 'id', // Optional (uses `mayors` primary key by default)
  });
});

// ... and the other one.
agent.customizeCollection('cities', collection => {
  // ⚠️ Not 'OneToOne'
  collection.addManyToOneRelation('mayor', 'mayors', {
    foreignKey: 'mayor_id',
    foreignKeyTarget: 'id', // Optional (uses `mayors` primary key by default)
  });
});