This chapter explains the basic mapping of objects and properties. Mapping of associations will be covered in the next chapter "Association Mapping".

++ Mapping Drivers

Doctrine provides several different ways for specifying object-relational mapping metadata:

* Docblock Annotations
* XML
* YAML

This manual usually uses docblock annotations in all the examples that are spread throughout all chapters. There are dedicated chapters for XML and YAML mapping, respectively.

> **NOTE**
> If you're wondering which mapping driver gives the best performance, the answer is:
> None. Once the metadata of a class has been read from the source (annotations, xml or
> yaml) it is stored in an instance of the `Doctrine\ORM\Mapping\ClassMetadata` class
> and these instances are stored in the metadata cache. Therefore at the end of the day
> all drivers perform equally well. If you're not using a metadata cache (not
> recommended!) then the XML driver might have a slight edge in performance due to the
> powerful native XML support in PHP.

++ Introduction to Docblock Annotations

You've probably used docblock annotations in some form already, most likely to provide documentation metadata for a tool like `PHPDocumentor` (@author, @link, ...). Docblock annotations are a tool to embed metadata inside the documentation section which can then be processed by some tool. Doctrine 2 generalizes the concept of docblock annotations so that they can be used for any kind of metadata and so that it is easy to define new docblock annotations. In order to allow more involved annotation values and to reduce the chances of clashes with other docblock annotations, the Doctrine 2 docblock annotations feature an alternative syntax that is heavily inspired by the Annotation syntax introduced in Java 5.

The implementation of these enhanced docblock annotations is located in the `Doctrine\Common\Annotations` namespace and therefore part of the Common package. Doctrine 2 docblock annotations support namespaces and nested annotations among other things. The Doctrine 2 ORM defines its own set of docblock annotations for supplying object-relational mapping metadata.

> **NOTE**
> If you're not comfortable with the concept of docblock annotations, don't worry, as
> mentioned earlier Doctrine 2 provides XML and YAML alternatives and you could easily
> implement your own favourite mechanism for defining ORM metadata.

++ Persistent classes

In order to mark a class for object-relational persistence it needs to be designated as an entity. This can be done through the `@Entity` marker annotation.

    [php]
    /** @Entity */
    class MyPersistentClass
    {
        //...
    }

By default, the entity will be persisted to a table with the same name as the class name. In order to change that, you can use the `@Table` annotation as follows:

    [php]
    /**
     * @Entity
     * @Table(name="my_persistent_class")
     */
    class MyPersistentClass
    {
        //...
    }

Now instances of MyPersistentClass will be persisted into a table named `my_persistent_class`.

++ Doctrine Mapping Types

A Doctrine Mapping Type defines the mapping between a PHP type and an SQL type. All Doctrine Mapping Types that ship with Doctrine are fully portable between different RDBMS. You can even write your own custom mapping types that might or might not be portable, which is explained later in this chapter.

For example, the Doctrine Mapping Type `string` defines the mapping from a PHP string to an SQL VARCHAR (or VARCHAR2 etc. depending on the RDBMS brand). Here is a quick overview of the built-in mapping types:

* `string`: Type that maps an SQL VARCHAR to a PHP string.
* `integer`: Type that maps an SQL INT to a PHP integer.
* `smallint`: Type that maps a database SMALLINT to a PHP integer.
* `bigint`: Type that maps a database BIGINT to a PHP string.
* `boolean`: Type that maps an SQL boolean to a PHP boolean.
* `decimal`: Type that maps an SQL DECIMAL to a PHP double.
* `date`: Type that maps an SQL DATETIME to a PHP DateTime object.
* `time`: Type that maps an SQL TIME to a PHP DateTime object.
* `datetime`: Type that maps an SQL DATETIME/TIMESTAMP to a PHP DateTime object.
* `text`: Type that maps an SQL CLOB to a PHP string.
* `object`: Type that maps a SQL CLOB to a PHP object using `serialize()` and `unserialize()`
* `array`: Type that maps a SQL CLOB to a PHP object using `serialize()` and `unserialize()`

> **NOTE**
> Doctrine Mapping Types are NOT SQL types and NOT PHP types! They are mapping types
> between 2 types.

> **CAUTION**
> Mapping types are *case-sensitive*. For example, using a DateTime column will NOT match the datetime type
> that ships with Doctrine 2!

++ Property Mapping

After a class has been marked as an entity it can specify mappings for its instance fields. Here we will only look at simple fields that hold scalar values like strings, numbers, etc. Associations to other objects are covered in the chapter "Association Mapping".

To mark a property for relational persistence the `@Column` docblock annotation is used. This annotation usually requires at least 1 attribute to be set, the `type`. The `type` attribute specifies the Doctrine Mapping Type to use for the field. If the type is not specified, 'string' is used as the default mapping type since it is the most flexible.

Example:

    [php]
    /** @Entity */
    class MyPersistentClass
    {
        /** @Column(type="integer") */
        private $id;
        /** @Column(length=50) */
        private $name; // type defaults to string
        //...
    }

In that example we mapped the field `id` to the column `id` using the mapping type `integer` and the field `name` is mapped to the column `name` with the default mapping type `string`. As you can see, by default the column names are assumed to be the same as the field names. To specify a different name for the column, you can use the `name` attribute of the Column annotation as follows:

    [php]
    /** @Column(name="db_name") */
    private $name;

The Column annotation has some more attributes. Here is a complete list:

* `type`: (optional, defaults to 'string') The mapping type to use for the column.
* `name`: (optional, defaults to field name) The name of the column in the database.
* `length`: (optional, default 255) The length of the column in the database. (Applies only if a string-valued column is used).
* `unique`: (optional, default FALSE) Whether the column is a unique key.
* `nullable`: (optional, default FALSE) Whether the database column is nullable.
* `precision`: (optional, default 0) The precision for a decimal (exact numeric) column. (Applies only if a decimal column is used.)
* `scale`: (optional, default 0) The scale for a decimal (exact numeric) column. (Applies only if a decimal column is used.)

++ Custom Mapping Types

Doctrine allows you to create new mapping types. This can come in handy when you're missing a specific mapping type
or when you want to replace the existing implementation of a mapping type.

In order to create a new mapping type you need to subclass `Doctrine\DBAL\Types\Type` and implement/override
the methods as you wish. Here is an example skeleton of such a custom type class:

    [php]
    namespace My\Project\Types;

    use Doctrine\DBAL\Types\Type;
    use Doctrine\DBAL\Platforms\AbstractPlatform;

    /**
     * My custom datatype.
     */
    class MyType extends Type
    {
        const MYTYPE = 'mytype'; // modify to match your type name

        public function getSqlDeclaration(array $fieldDeclaration, AbstractPlatform $platform)
        {
            // return the SQL used to create your column type. To create a portable column type, use the $platform.
        }

        public function convertToPHPValue($value, AbstractPlatform $platform)
        {
            // This is executed when the value is read from the database. Make your conversions here, optionally using the $platform.
        }

        public function convertToDatabaseValue($value, AbstractPlatform $platform)
        {
            // This is executed when the value is written to the database. Make your conversions here, optionally using the $platform.
        }

        public function getName()
        {
            return self::MYTYPE; // modify to match your constant name
        }
    }

Restrictions to keep in mind:

* If the value of the field is *NULL* the method `convertToDatabaseValue()` is not called.
* The `UnitOfWork` never passes values to the database convert method that did not change in the request.

When you have implemented the type you still need to let Doctrine know about it. This can be achieved
through the `Doctrine\DBAL\Configuration#setCustomTypes(array $types)` method.

> **NOTE**
> `Doctrine\ORM\Configuration` is a subclass of `Doctrine\DBAL\Configuration`, so the
> methods are available on your ORM Configuration instance as well.

Here is an example:

    [php]
    // in bootstrapping code

    // ...

    use Doctrine\DBAL\Types\Type;

    // ...

    // Register my type
    Type::addType('mytype', 'My\Project\Types\MyType');

As can be seen above, when registering the custom types in the configuration you specify a unique name
for the mapping type and map that to the corresponding fully qualified class name. Now you can use your new type in your mapping like this:

    [php]
    class MyPersistentClass
    {
        /** @Column(type="mytype") */
        private $field;
    }

To have Schema-Tool convert the underlying database type of your new "mytype" directly into an instance of `MyType`
you have to additionally register this mapping with your database platform:

    [php]
    $conn = $em->getConnection();
    $conn->getDatabasePlatform()->registerDoctrineTypeMapping('db_mytype', 'mytype');

Now using Schema-Tool, whenever it detects a column having the `db_mytype` it will convert it into a `mytype`
Doctrine Type instance for Schema representation. Keep in mind that you can easily produce clashes this way,
each database type can only map to exactly one Doctrine mapping type.

++ Identifiers / Primary Keys

Every entity class needs an identifier/primary key. You designate the field that serves as the identifier with the `@Id` marker annotation. Here is an example:

    [php]
    class MyPersistentClass
    {
        /** @Id @Column(type="integer") */
        private $id;
        //...
    }

Without doing anything else, the identifier is assumed to be manually assigned. That means your code would need to properly set the identifier property before passing a new entity to `EntityManager#persist($entity)`.

A common alternative strategy is to use a generated value as the identifier. To do this, you use the `@GeneratedValue` annotation like this:

    [php]
    class MyPersistentClass
    {
        /**
         * @Id @Column(type="integer")
         * @GeneratedValue
         */
        private $id;
    }

This tells Doctrine to automatically generate a value for the identifier. How this value is generated is specified by the `strategy` attribute, which is optional and defaults to 'AUTO'. A value of `AUTO` tells Doctrine to use the generation strategy that is preferred by the currently used database platform. See below for details.

+++ Identifier Generation Strategies

The previous example showed how to use the default identifier generation strategy without knowing the underlying database with the AUTO-detection strategy.
It is also possible to specifiy the identifier generation strategy more explicitly, which allows to make use of some additional features.

Here is the list of possible generation strategies:

* `AUTO` (default): Tells Doctrine to pick the strategy that is preferred by the used database platform.
   The preferred strategies are IDENTITY for MySQL, SQLite and MsSQL and SEQUENCE for Oracle and PostgreSQL.
   This strategy provides full portability.
* `SEQUENCE`: Tells Doctrine to use a database sequence for ID generation. This strategy does currently not provide full portability. Sequences are supported by Oracle and PostgreSql.
* `IDENTITY`: Tells Doctrine to use special identity columns in the database that generate a value on insertion of a row. This strategy does currently not provide full portability and
              is supported by the following platforms: MySQL/SQLite (AUTO_INCREMENT), MSSQL (IDENTITY) and PostgreSQL (SERIAL).
* `TABLE`: Tells Doctrine to use a separate table for ID generation. This strategy provides full portability. ***This strategy is not yet implemented!***
* `NONE`: Tells Doctrine that the identifiers are assigned (and thus generated) by your code.
          The assignment must take place before a new entity is passed to `EntityManager#persist`.
          NONE is the same as leaving off the @GeneratedValue entirely.


++++ Sequence Generator

The Sequence Generator can currently be used in conjunction with Oracle or Postgres and allows some additional configuration options besides
specifiying the sequence's name:

    [php]
    class User {
        /**
         * @Id
         * @GeneratedValue(strategy="SEQUENCE")
         * @SequenceGenerator(name="tablename_seq", initialValue=1, allocationSize=100)
         */
        protected $id = null;
    }

The initial value specifies at which value the sequence should start.

The allocationSize is a powerful feature to optimize INSERT performance of Doctrine. The allocationSize specifies
by how much values the sequence is incremented whenever the next value is retrieved. If this is larger than 1 (one)
Doctrine can generate identifier values for the allocationSizes amount of entities. In the above example with
`allocationSize=100` Doctrine 2 would only need to access the sequence once to generate the identifiers for
100 new entities.

*The default allocationSize for a @SequenceGenerator is currently 10.*

> **CAUTION**
> The allocationSize is detected by SchemaTool and transformed into an "INCREMENT BY <allocationSize>" clause
> in the CREATE SEQUENCE statement. For a database schema created manually (and not SchemaTool) you have to
> make sure that the allocationSize configuration option is never larger than the actual sequences INCREMENT BY value,
> otherwise you may get duplicate keys.

> **TIP**
> It is possible to use strategy="AUTO" and at the same time specifying a @SequenceGenerator.
> In such a case, your custom sequence settings are used in the case where the preferred
> strategy of the underlying platform is SEQUENCE, such as for Oracle and PostgreSQL.


+++ Composite Keys

Doctrine 2 allows to use composite primary keys. There are however some restrictions oposed to using a single identifier.
The use of the `@GeneratedValue` annotation is only supported for simple (not composite) primary keys, which means
you can only use composite keys if you generate the primary key values yourself before calling `EntityManager#persist()`
on the entity.

To designate a composite primary key / identifier, simply put the @Id marker annotation on all fields that make up the primary key.

++ Quoting Reserved Words

It may sometimes be necessary to quote a column or table name because it conflicts with a reserved word of the particular RDBMS in use. This is often referred to as "Identifier Quoting". To let Doctrine know that you would like a table or column name to be quoted in all SQL statements, enclose the table or column name in backticks. Here is an example:

    [php]
    /** @Column(name="`number`", type="integer") */
    private $number;

Doctrine will then quote this column name in all SQL statements according to the used database platform.

> **CAUTION**
> Identifier Quoting is not supported for join column names or discriminator column names.

> **CAUTION**
> Identifier Quoting is a feature that is mainly intended to support legacy database
> schemas. The use of reserved words and identifier quoting is generally discouraged.
> Identifier quoting should not be used to enable the use non-standard-characters such
> as a dash in a hypothetical column `test-name`. Also Schema-Tool will likely have
> troubles when quoting is used for case-sensitivity reasons (in Oracle for example).