Django has not considered NULL to be equal to NULL for the purpose of uniqueness checks since ticket #9039 was fixed, see: http://code.djangoproject.com/ticket/9039 The issue here is that the normalized “blank” value for a form CharField is an empty string, not None. So if you leave the field blank, you get an empty string, not … Read more
If you want a real database default value, use columnDefinition: @Column(name = “myColumn”, nullable = false, columnDefinition = “int default 100”) Notice that the string in columnDefinition is database dependent. Also if you choose this option, you have to use dynamic-insert, so Hibernate doesn’t include columns with null values on insert. Otherwise talking about default … Read more
I saw Spring, JPA works around repositories (DAO layer: if I am not wrong). So I mean how it is different using ‘Spring JPA + Hibernate’ or only using ‘Hibernate’ directly? As you said, JPA is an specification while Hibernate is a particular implementation of that specification (these implementations are usually referred to as Providers). … Read more
Some notes: JDO and JPA are both specifications, not implementations. The idea is you can swap JPA implementations, if you restrict your code to use standard JPA only. (Ditto for JDO.) Hibernate can be used as one such implementation of JPA. However, Hibernate provides a native API, with features above and beyond that of JPA. … Read more
MySQL is an example of a relational database – you would use an ORM to translate between your objects in code and the relational representation of the data. Examples of ORMs are nHibernate, Entity Framework, Dapper and more… MongoDB is an example of a document database – you would use an ODM to translate between … Read more
The main differenece is that bidirectional relationship provides navigational access in both directions, so that you can access the other side without explicit queries. Also it allows you to apply cascading options to both directions. Note that navigational access is not always good, especially for “one-to-very-many” and “many-to-very-many” relationships. Imagine a Group that contains thousands … Read more
Laravel 4+ Note for Laravel 5 users: You’ll need to call DB::enableQueryLog() before executing the query. Either just above the line that runs the query or inside a middleware. In Laravel 4 and later, you have to call DB::getQueryLog() to get all ran queries. $queries = DB::getQueryLog(); $last_query = end($queries); Or you can download a … Read more
You created a foreign key on Choice which relates each one to a Question. So, each Choice explicitly has a question field, which you declared in the model. Django’s ORM follows the relationship backwards from Question too, automatically generating a field on each instance called foo_set where Foo is the model with a ForeignKey field … Read more
Say you have a parent and that parent has a collection of children. Hibernate now can “lazy-load” the children, which means that it does not actually load all the children when loading the parent. Instead, it loads them when requested to do so. You can either request this explicitly or, and this is far more … Read more
According to JPA Spec: If an entity instance is to be passed by value as a detached object (e.g., through a remote interface), the entity class must implement the Serializable interface. “JSR 220: Enterprise JavaBeansTM,Version 3.0 Java Persistence API Version 3.0, Final Release May 2, 2006”