HasOne

HasOne is a one-to-one association used in databases, ORMs, and data modeling. It denotes that a single record in the source model is linked to at most one record in the related model. In most implementations the link is established by a foreign key in the related table pointing to the source table.

Conceptually, hasOne differs from belongsTo in the placement of the foreign key. In a hasOne relationship, the foreign key typically resides on the related model, while belongsTo places the foreign key on the source model. This distinction affects how queries are written and how deletions cascade.

Example in Laravel's Eloquent: class User extends Model { public function profile() { return $this->hasOne(Profile::class); } } class Profile extends Model { public function user() { return $this->belongsTo(User::class); } } In this setup the profiles table contains a user_id column referencing users.id. Likewise, in Rails you would declare has_one :profile on the User model and have profiles.user_id as the foreign key.

Database schema and constraints: a hasOne relation is implemented by adding a unique foreign key constraint on the related table to enforce one-to-one, and optionally making the foreign key non-null if a related record is required. You typically create an index on the foreign key for performance.

Performance and usage considerations: hasOne is commonly retrieved via eager loading to minimize queries (for example, in Laravel: with('profile')). Deleting a parent may require cascading delete or handling orphaned records depending on constraints. When optional, the related record may be null.

See also: hasMany, belongsTo, OneToOne (terminology varies across frameworks).