Monty's Blog

Another update to the database design, hopefully the final

2010-03-06T00:18:00.002Z

Nebeans: problem when pasring tables from mysq

2010-03-02T13:03:00.003Z

A weired thing happened to me today, as I created a new schema using MYSQL Workbench, and then tried to create entities from the database, netbeans complained that for some tables it cannot find the primary key, even it really exists. I googled around and found the solution is when forward engineering using MYSQL Woekbench you need to check(tick) the checbox for creating separate index statement as in the figure below:

Binding a JComboBox to a database query list

2010-02-28T22:22:00.006Z

exactly in the same way we bound the JTable, but we encounter one problem,

since each entity has more than one field/property, the combobox does not know

which to display so by default it uses the toString() method of each entity, which gives us

something displayed like this:

To change the display behaviour we need to customise the renderer of the Combobox

to render the required property of the entity as:

by getting the combo box propety we add code in the pre-init section as follows:

so we insert the following code:

Netbeans: Binding JTable to a database table

2010-02-25T22:59:00.005Z

To bind a JTable to a database table, you right click on the table
and select bind-> elements

then select import from Database and Netbeans will take care of creating EntityManager, a Query and a List with the result of the query.

The list is by default normal java.util.List so when it is modified the JTable which is bound to it does not get updated. To update it we need to set the List as Observale by right clicking on the list in the design view of netbeans and ticking the observable check box.

Updated Persistance Layer of the application Labels

2010-02-20T23:11:00.004Z

Finaly got to finalize the design of the persistence layer with the required business constraints such as: Table item: the combination of the columns-> name, packaging_packaging_id,category_category_id remains unique as we don't want to have an item with a size,name, category repeated more than once. so I have created an index called item_index to establish this purpose

Use of Generics to create a generic interface with common methods, for each class will have a different type

2010-02-19T22:13:00.002Z

Create a unique combination of columns in a table

2010-02-18T01:42:00.007Z

where upindex is the index name,
item is the table name is the columns are: name,packaging_packaging_id,category_category_id

Reset of auto_increment in mysql

2010-02-13T02:06:00.004Z

means next primary key will have the value 1
when using entity manager with transactions, even when you roll back the auto_increment valueproceeds so the next row will have a primary key with +1 even if it was rolled back!!

Creating OneToMany Uniderictional Relationship Example

2010-02-12T23:07:00.008Z

Lets take an example of the following Entities: Item, Ingredients
Item contains many ingredients so the relatioship between them is OneToMany.

Creating the Tables

Creating the Tables

The presence of the foreign key in the target table is indicating a one to many relatioship

now we need to map it in the two entity classes
in the item class
we create a List as follows:

and in the ingredient class we create an attribite corresponding
to the column of the foreign key as:

Now when we persists an Item object, automatically the item's primary key is created by the database as from 1 increments
then to map the ingredients to this entity(add them to this entity
we have to follow these steps
1- Retrieve the entity object from the database using

or using JPQL
2 - then creating the new ingredients entities
1- Retrieve the entity object from the database using

3 - then setting the relationship by setting the foreign key value as

and then calling a merge on item as

to update the item

GWT and JPA don't get along

2010-02-08T00:42:00.001Z

After doing a little research, for my work project Labels I discovered that I will have difficulties using JPA with GWT so I diceded to use Vaadin framework to create the presentation layer.

Creating My First Java EE Aplication

2009-12-31T23:49:00.008Z

Finally got time to fix the database at work that I have created(quickly) and inefficiently. The current database consists of 5 tables of identical structure, they all hold the same items but each table is for a different category.

This model is not good for many reasons, one is creating an item which already exists but with different size/packaging means duplicating the same information in a new row with the name change to accommodate the size. This is a text book redundancy of data, now i can use this bad model to create a more and better database structure and build on top of it a Java EE application using Java Persistance and EJB.

The Database Design phase:

After elaboration in which tables should fit the attributes of the original big table I came up with the following design.

So from the diagram we can see that the item name is used as a primary key in combination with the packaging which means that an item y with packaging x can only exists only once which we want to achive.
- scan_id can be null which will allow us to create item with no bar code
- cat_id cannot be null which will not allow us to create item without a category
- cok_id can be null, which will allow us to create item with no cooking instructions
- expiry field/column can be default 0, to enable us later to set the expiry duration for the items by the labelling software

Designing JPA Enities