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

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).

The idea behind the party models (aka entity schema) is to define a database that leverages some of the scalability benefits of schema-free databases. The party model does that by defining its entities as party type records, as opposed to one table per entity. The result is an extremely normalized database with very few tables and very little knowledge about the semantic meaning of the data it stores. All that knowledge is pushed to the data access in code. Database upgrades using the party model are minimal to none, since the schema never changes. It’s essentially a glorified key-value pair data model structure with some fancy names and a couple of extra attributes.

Pros:

• Kick-ass horizontal scalability. Once your 5-6 tables are defined in your entity model, you can go to the beach and sip margaritas. You can virtually scale this database out as much as you want with minimum efforts.
• The database supports any data structure you throw at it. You can also change data structures and party/entities definitions on the fly without affecting your application. This is very very powerful.
• You can model any arbitrary data entity by adding records, not changing the schema. Meaning you can say goodbye to schema migration scripts.
• This is programmers’ paradise, since the code they write will define the actual entities they use in code, and there are no mappings from Objects to Tables or anything like that. You can think of the Party table as the base object of your framework of choice (System.Object for .NET)

Cons:

• Party/Entity models never play well with ORMs, so forget about using EF or NHibernate to get semantically meaningful entities out of your entity database.
• Lots of joins. Performance tuning challenges. This ‘con’ is relative to the practices you use to define your entities, but is safe to say that you’ll be doing a lot more of those mind-bending queries that will bring you nightmares at night.
• Harder to consume. Developers and DB pros unfamiliar with your business will have a harder time to get used to the entities exposed by these models. Since everything is abstract, there no diagram or visualization you can build on top of your database to explain what is stored to someone else.
• Heavy data access models or business rules engines will be needed. Basically you have to do the work of understanding what the heck you want out of your database at some point, and your database model is not going to help you this time around.

If you are considering a party or entity schema in a relational database, you should probably take a look at other solutions like a NoSql data store, BigTable or KV Stores. There are some great products out there with massive deployments and traction such as MongoDB, DynamoDB, and Cassandra that pioneered this movement.

When I was part of a team implementing these ideas in the early 1980's, it did not limit our choice of ORM's because those hadn't been invented yet.

I'd fall back on those ideas any time, as that particular project was one of the most convincing proofs-of-concept I have ever seen of a "revolutionary" idea (which it certainly was at the time).

It forces you to nothing. And it doesn't stop you from anything (from any mistake, I mean). The one defining your own information model is you.

All parties have lots of properties in common. The fact that they have a name and such (we called those "signaletics"). The fact that they have principal/primary locations called "addresses". The fact that they all are involved, in some sense, in the business' contracts.

This is a vast topic, I would recommend reading The Data Model Resource Book Volume 3 - Universal Patterns for Data Modeling by Len Silverston and Paul Agnew.

I've just received my copy and it's pretty good - It provides you with an overlook for many approaches to data modeling, including hybrid contextual role patterns and so on. It has detailed PROs and CONs for every approach.

There is a pletheora of ways to model party relationships and roles all with their benefits and disadvantages. The question that was accepted as an answer covers just one instance of a 'party model'.

For instance, in many approaches, notions like "Employee", "Project Manager" etc. are roles that a party can play within a certain context. I will try to give you a better breakdown once I get home.