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. Sep 14, 2016 Use HiLo to generate keys with Entity Framework Core. To begin with, a little info about HiLo Pattern. HiLo is a pattern where the primary key is made of 2 parts “Hi” and “Lo”. Where the “Hi” part comes from database and “Lo” part is generated in memory to create unique value. Remember, “Lo” is a range number like 0-100.
Value generation patterns
There are three value generation patterns that can be used for properties:
No value generation
No value generation means that you will always supply a valid value to be saved to the database. This valid value must be assigned to new entities before they are added to the context.
Value generated on add
Value generated on add means that a value is generated for new entities.
Depending on the database provider being used, values may be generated client side by EF or in the database. If the value is generated by the database, then EF may assign a temporary value when you add the entity to the context. This temporary value will then be replaced by the database generated value during
SaveChanges() .
If you add an entity to the context that has a value assigned to the property, then EF will attempt to insert that value rather than generating a new one. A property is considered to have a value assigned if it is not assigned the CLR default value (
null for string , 0 for int , Guid.Empty for Guid , etc.). For more information, see Explicit values for generated properties.
Warning
How the value is generated for added entities will depend on the database provider being used. Database providers may automatically setup value generation for some property types, but others may require you to manually setup how the value is generated.
For example, when using SQL Server, values will be automatically generated for
GUID properties (using the SQL Server sequential GUID algorithm). However, if you specify that a DateTime property is generated on add, then you must setup a way for the values to be generated. One way to do this, is to configure a default value of GETDATE() , see Default Values.
Value generated on add or update
Value generated on add or update means that a new value is generated every time the record is saved (insert or update).
Like
value generated on add , if you specify a value for the property on a newly added instance of an entity, that value will be inserted rather than a value being generated. It is also possible to set an explicit value when updating. For more information, see Explicit values for generated properties.
Warning
How the value is generated for added and updated entities will depend on the database provider being used. Database providers may automatically setup value generation for some property types, while others will require you to manually setup how the value is generated.
For example, when using SQL Server,
byte[] properties that are set as generated on add or update and marked as concurrency tokens, will be setup with the rowversion data type - so that values will be generated in the database. However, if you specify that a DateTime property is generated on add or update, then you must setup a way for the values to be generated. One way to do this, is to configure a default value of GETDATE() (see Default Values) to generate values for new rows. You could then use a database trigger to generate values during updates (such as the following example trigger).
Value generated on add
By convention, non-composite primary keys of type short, int, long, or Guid are set up to have values generated for inserted entities, if a value isn't provided by the application. Your database provider typically takes care of the necessary configuration; for example, a numeric primary key in SQL Server is automatically set up to be an IDENTITY column.
You can configure any property to have its value generated for inserted entities as follows:
Warning
This just lets EF know that values are generated for added entities, it does not guarantee that EF will setup the actual mechanism to generate values. See Value generated on add section for more details.
Default values
On relational databases, a column can be configured with a default value; if a row is inserted without a value for that column, the default value will be used.
You can configure a default value on a property:
You can also specify a SQL fragment that is used to calculate the default value:
Specifying a default value will implicitly configure the property as value generated on add.
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Value generated on add or update![]()
Warning
This just lets EF know that values are generated for added or updated entities, it does not guarantee that EF will setup the actual mechanism to generate values. See Value generated on add or update section for more details.
Computed columns
On some relational databases, a column can be configured to have its value computed in the database, typically with an expression referring to other columns:
Note
In some cases the column's value is computed every time it is fetched (sometimes called virtual columns), and in others it is computed on every update of the row and stored (sometimes called stored or persisted columns). This varies across database providers.
No value generation
Disabling value generation on a property is typically necessary if a convention configures it for value generation. For example, if you have a primary key of type int, it will be implicitly set configured as value generated on add; you can disable this via the following:
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. 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:
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.
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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.
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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.
Unique KeyOutputEntity Framework Example
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 usedEntity Framework Uniqueidentifier
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
Above output shows that a no sequence or extra table were generated.
Entity Framework Guid Id GenerationExample Project
Dependencies and Technologies Used:
Entity Framework Guid Primary Key Auto Generated DownloadComments are closed.
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