You can’t have MySQL do this for you automatically for InnoDB tables – you would need to use a trigger or procedure, or use another DB engine such as MyISAM. Auto incrementing can only be done for a single primary key. Something like the following should work DELIMITER $$ CREATE TRIGGER xxx BEFORE INSERT ON … Read more
Add a unique index that includes both columns. That will prevent you from inserting a record that contains a duplicate category_id/post_id pair. add_index :categories_posts, [ :category_id, :post_id ], :unique => true, :name => ‘by_category_and_post’
If the class is not static, it requires an instance of the outer class in order to be instantiated – so there will be no default constructor. You’d have to use syntax similar to: new Supply().new Id(); If the Id class is static, you can just call: new Id();
The Manning book Java Persistence with Hibernate has an example outlining how to do this in Section 7.2. Fortunately, even if you don’t own the book, you can see a source code example of this by downloading the JPA version of the Caveat Emptor sample project (direct link here) and examining the classes Category and … Read more
Create a unique key on those columns ALTER TABLE YourTable add CONSTRAINT YourTable_unique UNIQUE (B, C, D); Oracle/PLSQL: Unique Constraints
They discourage them for several reasons: they’re cumbersome to use. Each time you need to reference an object (or row), for eexample in your web application, you need to pass 3 parameters instead of just one. they’re inefficient. Instead of simply hashing an integer, the database needs to hash a composite of 3 columns. they … Read more
You could also use HasKey(u => new { u.SubscriptionID, u.UserName }); Edit: One limitation I have found is that the following do not work: public ProjectAssignmentConfiguration() { HasKey(u => u.Employee.EmployeeId); HasKey(u => u.Project.ProjectId); } or public ProjectAssignmentConfiguration() { HasKey(u => new { u.Employee.EmployeeId, u.Project.ProjectId }); } So how do you set up an entity where … Read more
INSERT and UPDATE performance varies little: it will be almost same for (INT) and (INT, INT) keys. SELECT performance of composite PRIMARY KEY depends on many factors. If your table is InnoDB, then the table is implicitly clustered on the PRIMARY KEY value. That means that searches for both values will be faster if the … Read more
It’s called a composite key. If you want to change your actual PK to a composite one, use Alter table <your table> drop PRIMARY KEY; Alter table <your table> drop COLUMN <your autoincremented column>; Alter table <your table> add [constraint <constraint name>] PRIMARY KEY (<col1>, <col2>); You can also just add a unique constraint (your … Read more
SELECT m FROM Machine m WHERE m.machinePK.machineId = 10