For example, Cat. You may declare a persistent class using the class element:. NET class name of the persistent class or interface , including its assembly name. Acceptable values include null and not null. You may specify the name of the class itself. See Section See Section 5. It also allows entity mappings that are represented by dictionaries at the. Net level. In these cases, you should provide an explicit arbitrary name for the entity.
See Section 4. This is useful if you want to have a view instead of a base table. Valid values are none , drop , update , export , validate , all and any combination of them separated by commas ,. Not specifying it is treated as all. The node attribute is present on many more mapping elements, and has no usage on them too. It will be omitted from the documentation of these other elements.
It is perfectly acceptable for the named persistent class to be an interface. You may persist any inner class. You should specify the class name using the standard form ie.
This allows NHibernate to make some minor performance optimizations. The optional proxy attribute enables customization of the generated proxy for lazy initialization of persistent instances of the class: by default, the proxy derives from the class.
Specifying an interface causes the proxy to implement it and to use System. Object as its base class. This enables proxifying a sealed class or a class having non virtual members.
The specified interface should declare all the public members of the persistent class. On ISession. Load and on many-to-one associations, NHibernate will initially return proxies. The actual persistent object will be loaded when a method or property of the proxy is invoked.
Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or the class and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphism is useful when two different classes are mapped to the same table this allows a "lightweight" class that contains a subset of the table columns.
The persister attribute lets you customize the persistence strategy used for the class. You may, for example, specify your own subclass of NHibernate. EntityPersister or you might even provide a completely new implementation of the interface NHibernate. IClassPersister that implements persistence via, for example, stored procedure calls, serialization to flat files or LDAP. See NHibernate. CustomPersister for a simple example of "persistence" to a Hashtable.
These settings may increase performance in some cases, but might actually decrease performance in others. Use judiciously. Use of select-before-update will usually decrease performance. It is very useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to an ISession.
If you enable dynamic-update , you will have a choice of optimistic locking strategies:. This is the optimal strategy with respect to performance and is the only strategy that correctly handles modifications made outside of the session ie. Update is used. Beginning with NHibernate 1. This was done to allow using 0 as unsaved-value for the version property. An alternative to mapping a class to table or view columns is to map a query.
The content of the subselect element is a SQL query:. This ensures that auto-flush happens correctly and that queries against the derived entity do not return stale data. Mapped classes must declare the primary key column of the database table. Most classes will also have a property holding the unique identifier of an instance. See the generator element. Specifying either the attribute or the element is required.
The attribute takes precedence over the element. If the name attribute is missing, it is assumed that the class has no identifier property. The unsaved-value attribute is almost never needed in NHibernate. We strongly discourage its use for anything else. The required generator names a. NET class used to generate unique identifiers for instances of the persistent class.
All generators implement the interface NHibernate. This is a very simple interface; some applications may choose to provide their own specialized implementations.
However, NHibernate provides a range of built-in implementations. There are shortcut names for the built-in generators:. Do not use in a cluster. May generate colliding identifiers in a bit less than one year. The identifier returned by the database is converted to the property type using Convert. Any integral property type is thus supported. Do not use this generator with an user-supplied connection. Guid and its ToString string format method to generate identifiers of type string.
The length of the string returned depends on the configured format. Guid to create a byte[] that is converted to a string. Guid as the identifier. It is not equivalent to the Hibernate guid generator. Guid described by Jimmy Nilsson in this article. See also Section 5. Supports Section 5. The first implementation requires a "special" database table to hold the next available "hi" value. The second uses an Oracle-style sequence where supported. Unfortunately, you can't use hilo when supplying your own DbConnection to NHibernate.
NHibernate must be able to fetch the "hi" value in a new transaction. You can use the optional where parameter to specify the row to use in a table. This is useful if you want to use a single table for your identifiers, with different rows for each table. Do not include the where keyword in its value. This is not handled by NHibernate schema generation tooling. ToString format.
The valid values for format are described in the MSDN documentation. The default separator is - and should rarely be modified. The format determines if the configured separator can replace the default separator used by the format.
ToByteArray and then converting the byte[] into a char[]. The char[] is returned as a String consisting of 16 characters. The guid identifier is generated by calling Guid. To address some of the performance concerns with using Guids as primary keys, foreign keys, and as part of indexes with MS SQL the guid. The benefit of using the guid. Both these strategies require two SQL queries to insert a new object. For cross-platform development, the native strategy will choose from the identity , sequence and hilo strategies, dependent upon the capabilities of the underlying database.
If you want the application to assign identifiers as opposed to having NHibernate generate them , you may use the assigned generator. This special generator will use the identifier value already assigned to the object's identifier property. Be very careful when using this feature to assign keys with business meaning almost always a terrible design decision.
Due to its inherent nature, entities that use this generator may, when saved via the ISession's SaveOrUpdate method, cause NHibernate to query the database for checking if the entity exist or not. Depending on the version type, its default unsaved-value will generally already be suitable.
IsTransient for providing your own strategy for distinguishing newly instantiated objects. For assigned identifiers, unsaved-value default value is undefined.
NHibernate does not generate DDL with triggers. It is for legacy schemas only. In the above example, there is a unique valued property named socialSecurityNumber. It is defined by the class, as a property. Its value will be retrieved by querying the entity table filtered by the key property. If the class maps a natural id , key can be omitted. The natural id will be used when key is omitted. Starting with NHibernate release 3. The first aspect is database portability; the second is optimization.
Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above. The first of these new generators is NHibernate. SequenceStyleGenerator short name enhanced-sequence which is intended, firstly, as a replacement for the sequence generator and, secondly, as a better portability generator than native. This is because native generally chooses between identity and sequence which have largely different semantics that can cause subtle issues in applications eyeing portability.
SequenceStyleGenerator , however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used.
The difference between this and native is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what NHibernate tries to emulate with its table-based generators.
This generator has a number of configuration parameters:. The second of these new generators is NHibernate. TableGenerator short name enhanced-table , which is intended, firstly, as a replacement for the table generator, even though it actually functions much more like org.
Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This is the value which identifies which increment value to use. For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value.
Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators Section 5.
The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number".
Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. For a table with a composite key, you may map multiple properties of the class as identifier properties. Your persistent class must override Equals and GetHashCode to implement composite identifier equality.
It must also be marked with the Serializable attribute. Unfortunately, this approach to composite identifiers means that a persistent object is its own identifier. There is no convenient "handle" other than the object itself. You must instantiate an instance of the persistent class itself and populate its identifier properties before you can Load the persistent state associated with a composite key. We call this approach an embedded composite identifier, and discourage it for serious applications.
In other words, the instance will be saved inserted in the database. In other words, the instance will be updated. A much more convenient approach is to implement the composite identifier as a separate class, as in Section 8.
The attributes described below apply only to this alternative approach:. The second approach, an identifier component, is recommended for almost all applications. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. Allows content-based discrimination. The force attribute is only useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This will not usually be the case.
Using the formula attribute, you can declare an arbitrary SQL expression that will be used to evaluate the type of a row:. This is particularly useful if you plan to use long transactions see Section The identifier check on its own unsaved-value will take precedence, unless its own value is undefined. See the discussion of generated properties. NET 2. Any type implementing IVersionType is usable as a version. This is intended as an alternative to versioning.
Timestamps are by nature a less safe implementation of optimistic locking. However, sometimes the application might use the timestamps in other ways. NET type DateTime of the persistent class. It should not to be confused with source value db , which mandates NHibernate to retrieve the timestamp from the database in a dedicated query, in order to get the new value to set in the next insert or update.
From the database, or from the current runtime? Database-based timestamps incur an overhead because NHibernate must hit the database in order to determine the "next value". It is safer to use in clustered environments. Not all dialects are known to support the retrieval of the database's current timestamp.
Others may also be unsafe for usage in locking due to lack of precision. Setting both to false allows a pure "derived" property whose value is initialized from some other property that maps to the same column s or by a trigger or other application.
Computed properties do not have a column mapping of their own. In other words, determines if a version increment should occur when this property is dirty. A lazy property is not loaded when the object is initially loaded, unless the fetch mode has been overridden in a specific query. Values for lazy properties are loaded per lazy-group. Having lazy properties causes instances of the entity to be loaded as proxies. Theses proxies ignore the class proxy setting and always derives from the persistent class, requiring its members to be overridable.
When a lazy property is accessed, the other lazy properties of the lazy group are also loaded with it. Use unique-key instead if the value is unique only in combination with other properties. The column will be included in the index, along with other columns sharing the same unique-key logical name.
The actual index name depends on the dialect. The column will be included in the index, along with other columns sharing the same index logical name.
The name of a NHibernate basic type eg. The name of a. NET type with a default basic type eg. Int16, System. Single, System. Char, System. String, System. DateTime, System. The name of an enumeration type eg. Color, Eg. The class name of a custom type eg. NHibernate supports. These types are mostly treated the same as plain non- Nullable types internally.
If you do not specify a type, NHibernate will use reflection upon the named property to take a guess at the correct NHibernate type. Mail us on [email protected] , to get more information about given services.
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Improve this question. Community Bot 1 1 1 silver badge. Add a comment. Active Oldest Votes. In general you are right - as stated in documentation: 5.
Mapping one to one and one to many associations Small cite We can do that by explicit JOIN
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