OnetoMany

A one-to-many relationship is a type of data model relationship in databases and information systems in which a single record in one table (the one side) relates to zero or more records in another table (the many side). The relationship is typically realized by a foreign key on the many side that references the primary key of the one side.

Common examples include a customer and their orders, an author and their books, or a category and the products that belong to it. In relational databases, the foreign key constraint enforces that every record in the many-side table refers to an existing record in the one-side table, establishing referential integrity.

In application code and ORMs, the one-side entity exposes a collection of the many-side entities, while each many-side record holds a reference to the one-side entity. Database operations must consider cascading deletes or updates, and indexing the foreign key improves join performance. The relationship can be optional or required depending on whether the many side can exist without a related one-side row.

Performance considerations include the potential for N+1 query problems when loading the many-side collections lazily. Common strategies are eager loading, join queries, or batch loading. Normalization favors one-to-many over duplicating data, while denormalization may be used for read-heavy workloads.

Alternative terminology often describes the same concept in terms of the many-to-one perspective. Understanding the directionality is important for query design and ORM configuration.