在NHibernate中通过ID或父对象获取子对象？

Question

Let's say I have a Broker class which has a collection of Accounts . The Broker class maps to a database table called brokers , the Account class maps to a database table called accounts . ie

public class Broker
{
    public virtual IEnumerable<Account> Accounts
    {
        get { return _accounts; }
        protected set { _accounts = new HashSet<Account>(value); }
    }
    // other properties
}

public class Account 
{                
    public virtual Broker Broker { get; set; }
    // other properties
}

I want to write a method to return a list of accounts for a Broker and I'm not sure what the signature should be. I thought of two but not sure which to go for:

IList<Account> GetBrokerAccounts(Broker broker)

or

IList<Account> GetBrokerAccounts(int brokerId)

I'd be interested from a design point of view the pros and cons of each and what would be "best practise".

Edit

Here is my mapping file for my collection problem:

  <class name="MooDB.Domain.Broker,MooDB" table="brokers" >
    <id name="Id" column="brokerId" type="Int32" unsaved-value="0">
      <generator class="increment" />
    </id>
    <version name="Version" column="version" type="integer" unsaved-value="0" />
    <property name="Name" column="`name`" type="String" length="50" not-null="true" />
    <property name="IsActive" column="isActive" type="bool" not-null="true" />
    <property name="IsDefault" column="isDefault" type="bool" not-null="true" />
    <set name="BrokerInstruments" table="brokerInstruments" generic="true" inverse="true" lazy="true" cascade="delete">
      <key column="brokerId" />
      <one-to-many class="MooDB.Domain.BrokerInstrument" />
    </set>
    <set name="Accounts" table="accounts" generic="true" inverse="true" lazy="false" cascade="delete">
      <key column="accountId" />
      <one-to-many class="MooDB.Domain.Account,MooDB"/>
    </set>
  </class>

Answer 1

Not sure why you would need any of the two methods when you can already access a list of accounts directly from the Broker object.

var brokersAccounts = broker.Accounts;

NHibernate would already do the required work for you. In other words first find the broker object/entity and enumerate through its accounts.

Answer 2

Use the id, brokerId. It's a better practice (I don't believe in best practices) to use the simplest possible arguments and return types. Passing the id only also is typically much more useful in a stateless application, such as a web app.

Answer 3

Use The Load method. It creates proxy and thus you avoid loading of the Broker if you are only interested in the accounts.

public IList<Account> GetBrokerAccounts(int brokerId)
{
    return session.QueryOver<Account>()
    .Where(x => x.Broker == session.Load(brokerId)).List();
}

So if you get the brokerId from UI there is no need to load the Broker object from the database since you know it exists in the database. The code above will basically create select * from account where broker = brokerId type of query. If you would use Get it would generate two queries. One for loading the Broker and second to get the accounts.