Data Mapper and Relationships: Implementation strategies?

Question

I've almost finished my Data Mapper, but now I'm at the point where it comes to relationships.

I will try to illustrate my ideas here. I wasn't able to find good articles / informations on this topic, so maybe I'm re-inventing the wheel (for sure I am, I could just use a big framework - but I want to learn by doing it).

1:1 Relationships

First, lets look at 1:1 relationships. In general, when we've got an domain class called "Company" and one called "Address", our Company class will have something like address_id. Lets say in most cases we just display a list of Companies, and the address is only needed when someone looks at the details. In that case, my Data Mapper (CompanyDataMapper) simply loads lazily, meaning it will just fetch that address_id from the database, but will not do a join to get the address data as well.

In general, I have an getter method for every Relationship. So in this case, there's an getAddress(Company companyObject) method. It takes an company object, looks for it's address property and - if it's NULL - fetches the corresponding Address object from the database, using the Mapper class for that Address object (AddressDataMapper), and assigns that address object to the address property of the specified company object.

Important: Is a Data Mapper allowed to use another Data Mapper?

Lets say in most cases you need both the company object AND the address object, because you always display it in a list all together. In this case, the CompanyDataMapper not only fetches company objects, but does an SQL query with JOIN to also get all the fields of the address object. Finally, it iterates over the record set and feeds new objects with their corresponding values, assigning the address object to the company object.

Sounds simple, so far.

1:n Relationships

How about these? The only difference to 1:1 is, that an Company may have multiple Address objects. Lets have a look: When we're most of the time only interested in the Company, the Data Mapper would just set the addresses property of the company object to NULL. The addresses property is an array which may reference none, one or multiple addresses. But we don't know yet, since we load lazily, so it's just NULL. But what, if we would need all the addresses in most cases as well? If we would display a big list with all companys together with all their addresses? In this case, things start to get really ugly. First, we can't join the address table fifty times for every address object (I strongly believe that's impossible, and if it is, performance would be below zero). So, when we think this further down the road, it's impossible to NOT load lazily in this case.

Important: Is this true? Must I send out 100 queries to get 100 address objects, if I have 10 companies with each 10 addresses?

m:n Relationships

Lets say an address object only contains the country, state, city, road and house number. But one house could be a big business tower with lots of companies in them. Like one of those modern office buildings where anyone can rent a small rom to show off that tower on its website. So: Many companies can share the same address.

I have no plans yet to deal with that kind of problem.

Important: Probably it's not a bigger problem than the 1:n Relationships?

If anyone knows a good ressource that goes into details about solving / implementing this, I would be happy about a link!

Answer 1

I am looking forward to any answers you'll get on this topic, but in the meantime why not just hop over to Amazon (or your local books dealer) and finally buy

• Design Patterns: Elements of Reusable Object-Oriented Software, Gang of Four
• Patterns of Enterprise Architecture, Martin Fowler
• Domain-Driven Design: Tackling Complexity in the Heart of Software, Eric Evans

These book contain the original patterns you have been pointed at in various of your questions and are considered reference works in Design Patterns and Software Architecture.

Answer 2

I too am working through this issue. To start with, I have adapted the Data Mapper pattern from Matt Zandstra's PHP Objects, Patterns, and Practice (2d Ed). I see now that a new edition has come out

Perhaps the most ingenious part of the setup is the "Collections" objects. I am not sure what language you are using, so I'll spare you the details. Suffice it to say that PHP has an Iterator interface that makes it possible to load an array (map, in other languages) at first and transform the raw data into objects (hydrate?) on the fly, while looping.

Like you, I am struggling with how to load relationships. What I have found so far is that I can write my massive JOIN query in the Mapper class and create both a dehydrated collection for the target object and sneak in the data on the related objects at the same time.

I really dislike "Lazy Load" because it leads to so many database queries. It offends my perfectionist sensibilities to know that I am using tens or hundreds of queries to accomplish would could be done in one.

I, too, am looking forward to more answers.