Explain Eloquent relationship types, including polymorphic relationships.

Summary

One-to-one (hasOne/belongsTo), one-to-many (hasMany/belongsTo), many-to-many (belongsToMany with a pivot table), has-one/has-many-through, and polymorphic relationships where a model can belong to more than one other model type.

Longer Answer

• hasMany / belongsTo: the foreign key lives on the child (posts.user_id).
• belongsToMany: a pivot table (role_user); extra pivot columns with withPivot, timestamps with withTimestamps, a custom pivot model with using().
• hasManyThrough: reach distant records through an intermediate model (a country's posts through users).
• Polymorphic (morphTo / morphMany): one table serves several parents using commentable_type and commentable_id, for example comments on posts and videos. Many-to-many polymorphic uses morphToMany (tags).

Polymorphic relations cannot have real foreign-key constraints, so the database cannot enforce integrity. Use a morph map (Relation::enforceMorphMap) so class names are not stored in the database and refactors do not break data.

Key Points

• Know where the foreign key lives for each type.
• Pivot tables for many-to-many, with extra columns.
• Polymorphic: *_type + *_id, no FK constraints.
• Use an enforced morph map.

Follow-up questions

• What are the trade-offs of polymorphic relations versus separate tables?