- Values For The Primary Key Should Be Generated Automatically In Excel
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- Values For The Primary Key Should Be Generated Automatically Work
This feature is especially useful in the primary key field so that the key can be set automatically every time a new record is inserted. Although auto incrementing can be as simple as setting and forgetting it, there are times where you may want to manage the AUTOINCREMENT column to set the start number or perhaps skip certain values. We’ve already discussed a different kind of primary keys, either simple or composite primary keys have assigned an inline values in the executable applications that were made. By looking into @Id, @IdClass or @EmbeddedId examples you could see that. But what if we would have some generator to generate these primary values. You can then use the primary key to load the entity, and Hibernate sets the primary key value automatically. And if you want to persist a new entity, you need to set the primary key value programmatically. But with JPA and Hibernate you can do much more than that. You can: choose between different strategies to generate unique primary key values.
-->Value generation patterns
There are three value generation patterns that can be used for properties:
- No value generation
- Value generated on add
- Value generated on add or update
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
Values For The Primary Key Should Be Generated Automatically In Excel
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.
Value generated on add or update
Values For The Primary Key Should Be Generated Automatically Lyrics
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
Values For The Primary Key Should Be Generated Automatically Work
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: