How to use multiple primary keys

问题:

I created database, for my android app, witch has 16 tables. I want to use ORMlite mapping. The problem is that I didn't find examples where you have composite id(Multiple primary keys). For example I have table:

CREATE  TABLE IF NOT EXISTS `Tourist_Guide`.`Cultural_activity` (
  `City_Id` INT NOT NULL ,
  `activity_Id` INT NOT NULL ,
  `Cultural_activity_Id` INT NOT NULL AUTO_INCREMENT ,
  `Name_Of_Cultural_activity` VARCHAR(30) NOT NULL ,
  PRIMARY KEY (`Cultural_activity_Id`, `City_Id`, `activity_Id`) ,
  INDEX `fk_Cultural_activity_activity1` (`City_Id` ASC, `activity_Id` ASC) ,
  CONSTRAINT `fk_Cultural_activity_activity1`
    FOREIGN KEY (`City_Id` , `activity_Id` )
    REFERENCES `Tourist_Guide`.`activity` (`City_Id` , `activity_Id` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB;

Can you, please, tell me how to map this table to class(how this class should look like), is that even possible?

回答1:

Limitations

For simplicity, and to be able to have the same POCO class persisted in db4o, memcached, redis or on the filesystem (i.e. providers included in ServiceStack), each model must have a single primary key, by convention OrmLite expects it to be Id although you use [Alias("DbFieldName")] attribute it map it to a column with a different name or use the [PrimaryKey] attribute to tell OrmLite to use a different property for the primary key.

You can still SELECT from these tables, you will just be unable to make use of APIs that rely on it, e.g. Update or Delete where the filter is implied (i.e. not specified), all the APIs that end with ById, etc.

Workaround single Primary Key limitation

A potential workaround to support tables with multiple primary keys is to create an auto generated Id property that returns a unique value based on all the primary key fields, e.g:

public class OrderDetail
{
    public string Id { get { return this.OrderId + "/" + this.ProductId; } }



public int OrderId { get; set; }
    public int ProductId { get; set; }
    public decimal UnitPrice { get; set; }
    public short Quantity { get; set; }
    public double Discount { get; set; }
}

https://github.com/ServiceStack/ServiceStack.OrmLite/#limitations

回答2:

You have to use the following annotation above each unique field:

@DatabaseField (uniqueCombo = true)

Here are the docs on uniqueCombo.

回答3:

It is possible to generate an artificial ID field which consists of a composition of other fields. This can be done by settings the useSetGet Property in the @DatabaseField Annotation to true. This makes ORMLite call the getter and setter methods, instead of using reflection.

In your getter you could then return a composition of your fields.

This would look something like this in your example:

@DatabaseField(id=true, useGetSet=true)
private String id;

...

public String getId(){

    return culturalAcitivityId +"-" +cityId +"-" +activityId;

}
public void setId(String id){

    this.id = id;

}

http://ormlite.com/javadoc/ormlite-core/com/j256/ormlite/field/DatabaseField.html#useGetSet()

回答4:

ServiceStack's OrmLite by design doesn't support multiple composite primary keys. In order for your same POCOs to be useful outside of a db (e.g. NoSQL datastores, cache providers, etc) OrmLite expects a single primary key on each type, which my default is the Id property (overridable with ModelConfig class or [PrimaryKey], [Alias] attributes).

Work arounds to overcome this limitation include creating a single unique Id but include a unique constraint on the composite primary keys, e.g:

[CompositeIndex("CompositePkId1","CompositePkId2", Unique = true)] 
public class Poco 
{
    [AutoIncrement]
    public int Id { get; set; }
    public int CompositePkId1 { get; set; }
    public int CompositePkId2 { get; set; }
}

Or creating a pseudo column that contains combination of all primary keys, e.g:

public class Poco 
{
    public string Id { get { return CompositePkId1 + ":" + CompositePkId2; } }
    public int CompositePkId1 { get; set; }
    public int CompositePkId2 { get; set; }
}