What are the principles behind, and benefits of, the "party model"?

Question

The "party model" is a "pattern" for relational database design. At least part of it involves finding commonality between many entities, such as Customer, Employee, Partner, etc., and factoring that into some more "abstract" database tables.

I'd like to find out your thoughts on the following:

1. What are the core principles and motivating forces behind the party model?
2. What does it prescribe you do to your data model? (My bit above is pretty high level and quite possibly incorrect in some ways. I've been on a project that used it, but I was working with a separate team focused on other issues).
3. What has your experience led you to feel about it? Did you use it, and if so, would you do so again? What were the pros and cons?
4. Did the party model limit your choice of ORMs? For example, did you have to eliminate certain ORMs because they didn't allow for enough of an "abstraction layer" between your domain objects and your physical data model?

I'm sure every response won't address every one of those questions ... but anything touching on one or more of them is going to help me make some decisions I'm facing.

Thanks.

Answer 1

1. What are the core principles and motivating forces behind the party model?

To the extent that I've used it, it's mostly about code reuse and flexibility. We've used it before in the guest / user / admin model and it certainly proves its value when you need to move a user from one group to another. Extend this to having organizations and companies represented with users under them, and it's really providing a form of abstraction that isn't particularly inherent in SQL.

2. What does it prescribe you do to your data model? (My bit above is pretty high level and quite possibly incorrect in some ways. I've been on a project that used it, but I was working with a separate team focused on other issues).

You're pretty correct in your bit above, though it needs some more detail. You can imagine a situation where an entity in the database (call it a Party) contracts out to another Party, which may in turn subcontract work out. A party might be an Employee, a Contractor, or a Company, all subclasses of Party. From my understanding, you would have a Party table and then more specific tables for each subclass, which could then be further subclassed (Party -> Person -> Contractor).

3. What has your experience led you to feel about it? Did you use it, and if so, would you do so again? What were the pros and cons?

It has its benefits if you need flexibly to add new types to your system and create relationships between types that you didn't expect at the beginning and architect in (users moving to a new level, companies hiring other companies, etc). It also gives you the benefit of running a single query and retrieving data for multiple types of parties (Companies,Employees,Contractors). On the flip side, you're adding additional layers of abstraction to get to the data you actually need and are increasing load (or at least the number of joins) on the database when you're querying for a specific type. If your abstraction goes too far, you'll likely need to run multiple queries to retrieve the data as the complexity would start to become detrimental to readability and database load.

4. Did the party model limit your choice of ORMs? For example, did you have to eliminate certain ORMs because they didn't allow for enough of an "abstraction layer" between your domain objects and your physical data model?

This is an area that I'm admittedly a bit weak in, but I've found that using views and mirrored abstraction in the application layer haven't made this too much of a problem. The real problem for me has always been a "where is piece of data X living" when I want to read the data source directly (it's not always intuitive for new developers on the system either).