Entity Framework Guid Primary Key Auto Generated

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Entity Framework Guid Primary Key
Using Guids As Primary Key

Note: EF will throw an exception if you do not provide unique values each time because CourseId is a primary key property. Use the ValueGeneratedNever method of Fluent API to specify an Identity property in EF Core, as shown below. ModelBuilder.Entity.Property(p = p.CourseId).ValueGeneratedNever. Nov 19, 2015 One of the first steps in creating any database table is deciding what kind of data will uniquely identify a given row in said table; we call this a primary key.In modern databases, if we want to create a unique key from 'thin air' and not use already-existing data (this situation is known as using a surrogate key) we have two commonly-accepted options: using integers, or using Globally-Unique. The most frequent question asked by readers was: how to define an auto-increment Id field (or primary key) of an Entity? In other words, how can we tell the DB to assign an unique, incremental id number to each and every record by its own? Entity Framework Core Auto Generated guid asp.net-core-2.1 c# entity-framework-core. English (en). The primary key value needs to be specified even if it's usually generated by the database. It will be used to detect data changes between migrations. Entity Framework auto generate GUID. The migration generated doesn't include the default sql setting to the primary key as it should. Entity Framework Core. Data Annotations - Key Attribute in EF 6 & EF Core. The Key attribute can be applied to a property in an entity class to make it a key property and the corresponding column to a PrimaryKey column in the database. The default convention creates a primary key column for a property whose name is Id or Entity Class NameId. The Key attribute.

Nov 25, 2015 Auto-generated Guid in Entity Framework Code First When you create an entity in Code First model, you might need a Guid field. In addition you might want SQL Server should auto-generate values for that field.

A key serves as a unique identifier for each entity instance. Most entities in EF have a single key, which maps to the concept of a primary key in relational databases (for entities without keys, see Keyless entities). Entities can have additional keys beyond the primary key (see Alternate Keys for more information).

By convention, a property named Id or <type name>Id will be configured as the primary key of an entity.

Note

Owned entity types use different rules to define keys.

You can configure a single property to be the primary key of an entity as follows:

You can also configure multiple properties to be the key of an entity - this is known as a composite key. Composite keys can only be configured using the Fluent API; conventions will never setup a composite key, and you can not use Data Annotations to configure one.

Primary key name

By convention, on relational databases primary keys are created with the name PK_<type name>. You can configure the name of the primary key constraint as follows:

Key types and values

While EF Core supports using properties of any primitive type as the primary key, including string, Guid, byte[] and others, not all databases support all types as keys. In some cases the key values can be converted to a supported type automatically, otherwise the conversion should be specified manually.

Key properties must always have a non-default value when adding a new entity to the context, but some types will be generated by the database. In that case EF will try to generate a temporary value when the entity is added for tracking purposes. After SaveChanges is called the temporary value will be replaced by the value generated by the database.

Important

If a key property has its value generated by the database and a non-default value is specified when an entity is added, then EF will assume that the entity already exists in the database and will try to update it instead of inserting a new one. To avoid this turn off value generation or see how to specify explicit values for generated properties.

Alternate Keys

An alternate key serves as an alternate unique identifier for each entity instance in addition to the primary key; it can be used as the target of a relationship. When using a relational database this maps to the concept of a unique index/constraint on the alternate key column(s) and one or more foreign key constraints that reference the column(s).

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Tip

If you just want to enforce uniqueness on a column, define a unique index rather than an alternate key (see Indexes). In EF, alternate keys are read-only and provide additional semantics over unique indexes because they can be used as the target of a foreign key.

Alternate keys are typically introduced for you when needed and you do not need to manually configure them. By convention, an alternate key is introduced for you when you identify a property which isn't the primary key as the target of a relationship.

You can also configure a single property to be an alternate key:

You can also configure multiple properties to be an alternate key (known as a composite alternate key):

Finally, by convention, the index and constraint that are introduced for an alternate key will be named AK_<type name>_<property name> (for composite alternate keys <property name> becomes an underscore separated list of property names). You can configure the name of the alternate key's index and unique constraint:

Every JPA entity is required to have a field which maps to primary key of the database table. Such field must be annotated with @Id.

Simple vs Composite primary keys

A simple primary key consists of a single Java field which maps to a single table column.
A composite primary key consists of multiple Java fields which individually map to separate columns.

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Supported types for a primary key

A simple primary key field or one of the composite primary key field should be one of the following types:

Any Java primitive type
any Any primitive wrapper type
java.lang.String
java.util.Date
java.sql.Date
java.math.BigDecimal
java.math.BigInteger

In this tutorial we are going to focus on generation strategies of simple primary key.

@GeneratedValue Annotation

This annotation defines the types of primary key generation strategies. If this annotation is not used then application is responsible to populate and manage @Id field values itself.

The use of the GeneratedValue annotation is only required to be supported for simple primary keys.

GenerationType enum defines four strategies: Generation Type . TABLE, Generation Type. SEQUENCE, Generation Type. IDENTITY and Generation Type. AUTO. Let's understand them with examples.

GenerationType.SEQUENCE

With this strategy, underlying persistence provider must use a database sequence to get the next unique primary key for the entities.

We have created the following Util class to reuse the code for other examples.

Also, in the persistence.xml, we have created four persistence-unit, so that we can try four GenerationType independently. We are using Hibernate as persistence provider.

Let's create main class to try out Entity1 key generation.

Output

Above output shows one table MYENTITY1 and one sequence HIBERNATE_SEQUENCE are created.

GenerationType.TABLE

With this strategy, underlying persistence provider must use a database table to generate/keep the next unique primary key for the entities.

Output

This time no sequence is generated, instead an additional table named 'HIBERNATE_SEQUENCES' is created to maintain primary key sequence.

GenerationType.IDENTITY

This GenerationType indicates that the persistence provider must assign primary keys for the entity using a database identity column. IDENTITY column is typically used in SQL Server. This special type column is populated internally by the table itself without using a separate sequence. If underlying database doesn't support IDENTITY column or some similar variant then the persistence provider can choose an alternative appropriate strategy. In this examples we are using H2 database which doesn't support IDENTITY column.

Output

Above output shows that a sequence is used for primary keys.

GenerationType.AUTO

This GenerationType indicates that the persistence provider should automatically pick an appropriate strategy for the particular database. This is the default GenerationType, i.e. if we just use @GeneratedValue annotation then this value of GenerationType will be used.

Output

Above output shows that a sequence is used for primary keys.

When @GeneratedValue not used

If we don't use @GeneratedValue annotation at all, then we have to populate the unique primary keys ourselves. In this example, we are simply assigning it to the value returned from System.nanoTime()

Output