Subclass Entities

By default, Quick supports basic component-level inheritance of entities, meaning that a child component inherits the properties ( and ability to overload ) its parent. A common, object-oriented relational database pattern, however is to provide additional definition on parent tables ( and classes ) within child tables which contain a foreign key.

Quick supports two types of child classes: Discriminated and Subclassed child entities. In both cases, loading any child class will also deliver the data of its parent class.

Subclass Entities

Let's say, for example, that I have a Media entity, which is used to catalog and organize all media items loaded in to my application.

component 
    table="media" 
    extends="quick.models.BaseEntity" 
    accessors="true"
{

    property name="id";
    property name="uploadFileName";
    property name="fileLocation";
    property name="fileSizeBytes";

}

My Media entity contains all of the properties which are common to every media item uploaded in to my application. Let's say, however, that I need to have specific attributes that are available on only media for my Book entity ( e.g. whether the image is the cover photo, for example ). I can create a child class of BookMedia which extends my Media entity. When loaded, all of the properties of Media will be loaded along with the custom attributes which apply to my BookMedia object:

component
    extends="Media"
    table="book_media"
    joinColumn="FK_media"
    accessors="true"
{
    property name="displayOrder";
    property name="designation";

    function approvalStatus(){
        return belongsTo( "Book", "FK_book" );
    }

}

Note the additional component attribute joincolumn. The presence of this attribute on a child class signifies that it is a child entity of the parent and that the parent's properties should be loaded whenever the BookMedia entity is loaded. In addition, the primary key of the entity is that of the parent.

Note that a table attribute is required on a child entity if the parent entity has one. This is because ColdBox will perform a deep merge on the entire inheritance chain for metadata properties. If a parent class has a table attribute, it will show up as the child's table attribute.

Child entities can be retrieved by queries specific to their own properties:

var coverPhotos = getInstance( "BookMedia" )
                    .where( "designation", "cover" )
                    .orderBy( "displayOrder", "ASC" );

Or properties on the parent class can be used as first-class properties within the query:

var smallCoverPhotos = getInstance( "BookMedia" )
                    .where( "designation", "cover" )
                    .where( "fileSizeBytes", "<", 40000 )
                    .orderBy( "displayOrder", "ASC" )
                    .orderBy( "uploadFileName", "ASC" );

Child entities can be retrieved, individually, using the value of the joinColumn, which should be a foreign key to the parent identifier column:

var myBookMediaItem = getInstance( "BookMedia" ).get( myId );

Now my Book entity can use its extended media class to retrieve media items which are specific to its own purpose:

function media(){
    return hasMany( "BookMedia", "FK_book" ).orderBy( "displayOrder", "ASC" );
}

Discriminated Entities

A discriminated child class functions, basically, in the same way as a subclassed entity, with one exception: The parent entity is aware of the discriminated child, due to a discriminatorValue attribute and will return that specific class when a retrieval is performed through the parent Entity. Let's take our BookMedia class, again, but this time defining it as a discriminated entity.

The first step is to add the discriminatorColumn attribute to the Media entity:

component 
    table="media" 
    extends="quick.models.BaseEntity" 
    accessors="true"
    discriminatorColumn="type"
{

    property name="id";
    property name="uploadFileName";
    property name="fileLocation";
    property name="fileSizeBytes";

}

Then we set a discriminatorValue property on the child class, the value of which is stored in the parent entity table:

component
    extends="Media"
    table="book_media"
    joinColumn="FK_media"
    discriminatorValue="book"
    accessors="true"
{
    property name="displayOrder";
    property name="designation";

    function approvalStatus(){
        return belongsTo( "Book", "FK_book" );
    }

}

Finally, we define an array of possible discriminated entities for the parent. This is so we don't have to scan all Quick components just to determine if there are any discriminated entities.

component
    extends="Media"
    table="book_media"
    joinColumn="FK_media"
    discriminatorValue="book"
    accessors="true"
{
    property name="displayOrder";
    property name="designation";
    
    variables._discriminators = [
        "BookMedia"
    ];

    function approvalStatus(){
        return belongsTo( "Book", "FK_book" );
    }

}

Once this is defined, any new BookMedia entity will be saved with a type value of book in the media table. As such, the following query will result in only entities of BookMedia being returned:

var bookMedia = getInstance( "Media" ).where( "type", "book" ).get();

If our Media table contains a combination of non-book and book media, then the collection returned when querying all records will contain a mix of BookMedia and Media entities.

Loading a collection of BookMedia entities, however, will always return a collection of BookMedia entities, because the type column value on the media must be equal to book.

Summary

Discriminated and child class entities, allow for a more Object oriented approach to entity-specific relationships by allowing you to eliminate pivot/join tables and extend the attributes of the base class.