doctrine2/manual/en/inheritance-mapping.txt

++ Mapped Superclasses

An mapped superclass is an abstract or concrete class that provides persistent entity state and mapping information
for its subclasses, but which is not itself an entity. Typically, the purpose of such a mapped superclass is to define
state and mapping information that is common to multiple entity classes. 

Mapped superclasses, just as regular, non-mapped classes, can appear in the middle of an otherwise
mapped inheritance hierarchy (through Single Table Inheritance or Class Table Inheritance).

> **NOTE**
>
> A mapped superclass cannot be an entity, it is not queryable and persistent relationships defined by a mapped
> superclass must be unidirectional. For further support of inheritance, the single or joined table inheritance
> features have to be used.

Example:

    [php]
    /** @MappedSuperclass */
    class MappedSuperclassBase
    {
        /** @Column(type="integer") */
        private $mapped1;
        /** @Column(type="string") */
        private $mapped2;
        /**
         * @OneToOne(targetEntity="MappedSuperclassRelated1")
         * @JoinColumn(name="related1_id", referencedColumnName="id")
         */
        private $mappedRelated1;
        
        // ... more fields and methods
    }

    /** @Entity */
    class EntitySubClass extends MappedSuperclassBase
    {
        /** @Id @Column(type="integer") */
        private $id;
        /** @Column(type="string") */
        private $name;

        // ... more fields and methods
    }
    
The DDL for the corresponding database schema would look something like this (this is for SQLite):

    [sql]
    CREATE TABLE EntitySubClass (mapped1 INTEGER NOT NULL, mapped2 TEXT NOT NULL, id INTEGER NOT NULL, name TEXT NOT NULL, related1_id INTEGER DEFAULT NULL, PRIMARY KEY(id))
    
As you can see from this DDL snippet, there is only a single table for the entity subclass. All the mappings from the mapped superclass were inherited to the subclass as if they had been defined on that class directly.


++ Single Table Inheritance

[Single Table Inheritance](http://martinfowler.com/eaaCatalog/singleTableInheritance.html) is an inheritance mapping strategy where all classes of a hierarchy are mapped to a single database table. In order to distinguish which row represents which type in the hierarchy a so-called discriminator column is used.

Example:

    [php]
    namespace MyProject\Model;
    
    /**
     * @Entity
     * @InheritanceType("SINGLE_TABLE")
     * @DiscriminatorColumn(name="discr", type="string")
     * @DiscriminatorMap({"person" = "Person", "employee" = "Employee"})
     */
    class Person
    {
        // ...
    }
    
    /**
     * @Entity
     */
    class Employee extends Person
    {
        // ...
    }

Things to note:

* The @InheritanceType, @DiscriminatorColumn and @DiscriminatorMap must be specified on the topmost class that is part of the mapped entity hierarchy.
* The @DiscriminatorMap specifies which values of the discriminator column identify a row as being of a certain type. In the case above a value of "person" identifies a row as being of type `Person` and "employee" identifies a row as being of type `Employee`.
* The names of the classes in the discriminator map do not need to be fully qualified if the classes are contained in the same namespace as the entity class on which the discriminator map is applied.

+++ Design-time considerations

This mapping approach works well when the type hierarchy is fairly simple and stable. Adding a new type to the hierarchy and adding fields to existing supertypes simply involves adding new columns to the table, though in large deployments this may have an adverse impact on the index and column layout inside the database.

+++ Performance impact

This strategy is very efficient for querying across all types in the hierarchy or for specific types. No table joins are required, only a WHERE clause listing the type identifiers. In particular, relationships involving types that employ this mapping strategy are very performant.


++ Class Table Inheritance

[Class Table Inheritance](http://martinfowler.com/eaaCatalog/classTableInheritance.html) is an inheritance mapping strategy where each class in a hierarchy is mapped to several tables: its own table and the tables of all parent classes. The table of a child class is linked to the table of a parent class through a foreign key constraint.
Doctrine 2 implements this strategy through the use of a discriminator column in the topmost table of the hierarchy because this is the easiest way to achieve polymorphic queries with Class Table Inheritance.

Example:

    [php]
    namespace MyProject\Model;
    
    /**
     * @Entity
     * @InheritanceType("JOINED")
     * @DiscriminatorColumn(name="discr", type="string")
     * @DiscriminatorMap({"person" = "Person", "employee" = "Employee"})
     */
    class Person
    {
        // ...
    }
    
    /** @Entity */
    class Employee extends Person
    {
        // ...
    }

Things to note:

* The @InheritanceType, @DiscriminatorColumn and @DiscriminatorMap must be specified on the topmost class that is part of the mapped entity hierarchy.
* The @DiscriminatorMap specifies which values of the discriminator column identify a row as being of which type. In the case above a value of "person" identifies a row as being of type `Person` and "employee" identifies a row as being of type `Employee`.
* The names of the classes in the discriminator map do not need to be fully qualified if the classes are contained in the same namespace as the entity class on which the discriminator map is applied.

> **NOTE**
> When you do not use the SchemaTool to generate the required SQL you should know that deleting a class table inheritance
> makes use of the foreign key property `ON DELETE CASCADE` in all database implementations. A failure to implement this
> yourself will lead to dead rows in the database.

+++ Design-time considerations

Introducing a new type to the hierarchy, at any level, simply involves interjecting a new table into the schema. Subtypes of that type will automatically join with that new type at runtime. Similarly, modifying any entity type in the hierarchy by adding, modifying or removing fields affects only the immediate table mapped to that type. This mapping strategy provides the greatest flexibility at design time, since changes to any type are always limited to that type's dedicated table.

+++ Performance impact

This strategy inherently requires multiple JOIN operations to perform just about any query which can have a negative impact on performance, especially with large tables and/or large hierarchies. When partial objects are allowed, either globally or on the specific query, then querying for any type will not cause the tables of subtypes to be OUTER JOINed which can increase performance but the resulting partial objects will not fully load themselves on access of any subtype fields, so accessing fields of subtypes after such a query is not safe.