ActiveJDBC – as ORM :: More Examples

• ActiveJDBC More Examples

Some more concepts from ActiveJDBC

ActiveJDBC is lazy by default.

In this sense, it has semantics closer to ActiveRecord than Hibernate.

This means that if you have a model Passenger and a model Address,and they have a one to many relationship, when a passenger has many addresses, the code: does not load the associated addresses.

Passenger p = Passenger.findById(1);

Let’s consider an example where an ORM could unexpectedly generate a huge number of inefficient queries:

List<Address> addresses = Address.findAll();

 for(Address address: addresses){
    Passenger p = address.parent(Passenger.class);
    System.out.println(p);
}

In the above example, the number of queries generated and executed is going to be N + 1, were N is a number of addresses.

This is because the first query is to get all addresses, and then for each address, there is a new query to get a user parent (line 4).

This approach is going to kill performance in some applications.

A better approach is to load all parents at once by a single query:

List<Address> addresses = Address.findAll().include(Passenger.class);
 
 for(Address address: addresses){
    Passenger p = address.parent(Passenger.class);
    System.out.println(p);
}

The ActiveJDBC will then issue two queries:

one to get all Address(es) and the other to get all corresponding User(s).

The same logic can be applied to all relationships going up and down: one to many, many to one and many to many.

Eager simultaneous loading of parents and children

Suppose we have two one to many relationships:

Author has many Posts and a Post has many Comments.

In cases like these, we can load a post and all corresponding Authors and Comments very efficiently:

List<Post> todayPosts = Post.where("post_date = ?", today).include(Author.class, Comment.class);

Click the below Link to download complete Library and Examples

• ActiveJDBC Mapping Example using Ant and Eclipse(7 KB)
• ActiveJDBC Lazy Loading Example using Ant and Eclipse(7 KB)
• ActiveJDBC Auto Generated Field Example using Ant and Eclipse(7 KB)
• ActiveJDBC Eager Loading Example using Ant and Eclipse(7 KB)
• ActiveJDBC Batch Operation Example using Ant and Eclipse(7 KB)
• ActiveJDBC Transaction Example using Ant and Eclipse(7 KB)
• ActiveJDBC Convert Example using Ant and Eclipse(7 KB)
• ActiveJDBC with Web Application using Ant and Eclipse(7 KB)
• ActiveJDBC Limit and Sort Example using Ant and Eclipse(7 KB)
• First ActiveJDBC Basic Example using Ant and Eclipse(7 KB)
• All required JAR library (1.4 MB)