I am new to Object oriented design patterns. I have a basic idea of the domain model. However, I am stuck at the following problem. The problem is that I have a system that contains events and customers(both 1....*). Additionally, the events contain packages. The user is allowed to book events. However, he has to do it by booking a package that is contained inside an event. What should be the correct domain model of the given situation. I did a lot of research about similar questions, but could not find a suitable answer.
My guesses:
1)
In this image, The customers can book events, but I am unsure about this because customers are booking packages contained inside events. Should I select this domain model for the given system? It is intuitive because it allows customers to book events.
2)
In this image, the domain model signifies the system's structure. However, customers should reach the packages only through events. Therefore, I am unsure about this domain model. How can a customer book a package when it is contained inside an event.
Please suggest which domain model is correct. I am a beginner so please provide a good explanation. Thanks for helping me out!
First of all - there is no definite answer. Usually you can create in several different ways and have good results.
Second - both contain relationships should be aggregations in my opinion. Events to System is (probably) a composite aggregation while Package to Event can be either composite (if a particular Package is specific for one Event only) or shared (if the same Package is available through various Events).
Now to the main question. If the relationship between Event and Package is a composition (composite aggregation) then you can model book relationship as an association between Customer and Package. Then the Event is uniquely recognizable. You can also extend your model adding a relationship to the Event that is a derived one (from book).
Here is an example (note the / sign depicting this is a derived association):
Other option that is valid for both composite and shared aggregation between Event and Package is to model Book relationship between Customer and Event but model it as an association class. Then you have a class (Book) describing the association and this class can have a relationship chosenPackage to Package.
In this case your model will look like this:
I can bet that you can also find also other methods of modelling your problem that still are valid and show all the information you provided so far.
Related
I have seen an article in Dzone regarding Post and Post Details (two different entities) and the relations between them. There the post and its details are in different tables. But as I see it, Post Detail is an embeddable part because it cannot be used without the "parent" Post. So what is the logic to separate it in another table?
Please give me a more clear explanation when to use which one?
Embeddable classes represent the state of their parent classes. So to take your example, a StackOverflow POST has an ID which is invariant and used in an unbreakable URL for sharing e.g. http://stackoverflow.com/q/44017535/146325. There are a series of other attributes (state, votes, etc) which are scalar properties. When the post gets edited we have various versions of the text (which are kept and visible to people with sufficient rep). Those are your POST DETAILS.
"what is the logic to separate it in another table?"
Because keeping different things in separate tables is what relational databases do. The standard way of representing this data model is a parent table POST and child table POST_DETAIL with a defined relationship enforced through a foreign key.
Embeddable is a concept from object-oriented programming. Oracle does support object-relational constructs in the database. So it would be possible to define a POST_DETAIL Type and create a POST Table which has a column declared as a nested table of that Type. However, that would be a bad design for two reasons:
The SQL for working with nested tables is clunky. For instance, to get the POST and the latest version of its text would require unnesting the collection of details every time we need to display it. Computationally not much different from joining to a child table and filtering on latest version flag, but harder to optimise.
Children can have children themselves. In the case of Posts, Tags are details because they can vary due to editing. But if you embed TAG in POST_DETAIL embedded in POST how easy would it be to find all the Posts with an [oracle] tag?
This is the difference between Object-Oriented design and relational design.
OO is strongly hierarchical: everything is belongs to something and the way to get the detail is through the parent. This approach works well when dealing with single instances of things, and so is appropriate for UI design.
Relational prioritises commonality: everything of the same type is grouped together with links to other things. This approach is suited for dealing with sets of things, and so is appropriate for data management tasks (do you want to find all the employees who work in BERLIN or whose job is ENGINEER or who are managed by ELLIOTT?)
"give me a more clear explanation when to use which one"
Always store the data relationally in separate tables. Build APIs using OO patterns when it makes sense to do so.
I'm trying to create a test (examination) app. However, I'm having difficulty designing my models. Can somebody suggest a good UML diagram to accommodate the following features:
Hierarchy:
Module has Unit(s)
Unit has Exam(s)
Exam has Question(s)
Question types: (True or False, Identification, Multiple Choice, Matching Type)
So far this is my original design:
With this design I can't accommodate a 'Matching Type' kind of question. In addition, how can I persist my previous exams and easily check correct answers?
When you are not sure, how to connect two classes, think, if you should use one more class between them.
You can create class diagram like this (created with SPARX Enterprise architect)
But associations between classes depend on functional requirements. E.g. One Exam belongs only to 1 particular Unit in this example, but you may need Exam that can belong to 0..N different Units. So this is just an example.
Also the way how to persist and check answers depends on further functional requirements. The simplest thing would be to store the Student's Answers with a link to a particular Question.
Adding matching type question complicates things a bit, here are few ideas:
The model uses generalization - Option is an abstract class with two inherited classes ListOption and MatchingTypeOption. When you need to store details about answers the similar must be done with SelectedOption because you need to store selected pairs of options for matching type.
But you can also store just info was the answer correct - yes/no.
Hopefully, this fictitious example will illustrate my problem:
Suppose you are writing a system which tracks complaints for a software product, as well as many other attributes about the product. In this case the SoftwareProduct is our aggregate root and Complaints are entities that only can exist as a child of the product. In other words, if the software product is removed from the system, so shall the complaints.
In the system, there is a dashboard like web page which displays many different aspects of a single SoftwareProduct. One section in the dashboard, displays a list of Complaints in a grid like fashion, showing only some very high level information for each complaint. When an admin type user chooses one of these complaints, they are directed to an edit screen which allows them to edit the detail of a single Complaint.
The question is: what is the best way for the edit screen to retrieve the single Complaint, so that it can be displayed for editing purposes? Keep in mind we have already established the SoftwareProduct as an aggregate root, therefore direct access to a Complaint should not be allowed. Also, the system is using NHibernate, so eager loading is an option, but my understanding is that even if a single Complaint is eager loaded via the SoftwareProduct, as soon as the Complaints collection is accessed the rest of the collection is loaded. So, how do you get the single Complaint through the SoftwareProduct without incurring the overhead of loading the entire Complaints collection?
This gets a bit into semantics and religiosity, but within the context of editing a complaint, the complaint is the root object. When you are editing a complaint, the parent object (software product) is unimportant. It is obviously an entity with a unique identity. Therefore you would would have a service/repository devoted to saving the updated complaint, etc.
Also, i think you're being a bit too negative. Complaints? How about "Comments"? Or "ConstructiveCriticisms"?
#Josh,
I don't agree with what you are saying even though I have noticed some people design their "Web" applications this way just for the sake of performance, and not based on the domain model itself.
I'm not a DDD expert either, but I'm sure you have read the traditional Order and OrderItem example. All DDD books say OrderItem belongs to the Order aggregate with Order being the aggregate root.
Based on what you are saying, OrderItem doesn't belong to Order aggregate anymore since the user may want to directly edit an OrderItem with Order being unimportant (just like editing a Complaing with its parents Software Product being unimportant). And you know if this approach is followed, none of the Order invariants could be enforced, which are extremely important when it comes to e-commerce systems.
Anyone has any better approaches?
Mosh
To answer your question:
Aggregates are used for the purpose of consistency. For example, if adding/modifying/deleting a child object from a parent (aggregate root) causes an invariant to break, then you need an aggregate there.
However, in your problem, I believe SoftwareProduct and Compliant belong to two separate aggregates, each being the root of their own aggregates. You don't need to load the SoftwareProject and all N Complaints assigned to it, just to add a new Complaint. To me, it doesn't seem that you have any business rules to be evaluated when adding a new Complaint.
So, in summary, create 2 different Repositories: SoftwareProductRepository and ComplaintRepository.
Also, when you delete a SoftwareProduct, you can use database relationships to cascade deletes and remove the associated Complaints. This should be done in your database for the purpose of data integrity. You don't need to control that in your Domain Model, unless you had other invariants apart from deleting linked objects.
Hope this helps,
Mosh
I am using NH for another business context but similar entity relationships like yours. I do not understand why do you say:
Keep in mind we have already
established the SoftwareProduct as an
aggregate root, therefore direct
access to a Complaint should not be
allowed
I have this in mine, Article and Publisher entities, if Publisher cease to exist, so do all the dependent Artcle entities. I allow myself to have direct access to the Article collections of each Publisher and individual entities. In the DB/Mapping of the Article class, Publisher is one of the members and cannot accept Null.
Care to elaborate the difference between yours and mine?
Sorry this is not a direct answer but too long to be added as a comment.
I agree with Mosh. Each ones of these two entities has its own aggregate root. Let me to explain it in the real life. suppose that a company has developed a software. There are some bug in this software, that made you annoy. you are going to go to the company and aware them from this problem. this company gives you a form to be filled by you.
This form has a field - section - indicates to the software name and description. additionally, it has some parts for your complaint. Is this form the same as the software manual? No. It is a form related to the software. It is not the software. Does this form has any ID? yes. It has. In other words, you can call the company in the next day and ask the operator about your letter of complaint. It is obvious that the operator will ask you about the Id.
This evidence shows that this form has its own entity and it could not be confused with the software itself. Any relation between two different entity does not mean one of them belongs to the other.
Update Edited to reflect clarifications requested by Chris Holmes below. Originally I was referring to a Terminal as a Site but changed it to better reflect my actual domain.
At heart, I think this is a question about modeling a many to many relationship between two root entities where the relationship itself carries some semantic meaning.
In my domain
You can think of a Terminal as a branch location of our company
A Terminal can have a relationship with any number of customers
A customer can have a relationship with any number of terminals (standard many to many)
A customer\terminal relationship means that a customer can potentially store products at the Terminal
This relationship can be enabled\disabled. To be disabled merely means you are temporarily not allowed to store product, so a disabled relationship is different from no relationship at all.
A customer can have many Offices
A Terminal that has a relationship with a customer (enabled or not) must have a default office for that customer that they communicate with
There are some default settings that are applied to all transactions between a Customer and a Terminal, these are set up on a Terminal-Customer Relationship level
I think my objects here are pretty clear, Terminal, Customer, Office, and TerminalCustomerRelationship (since there is information being stored specifically about the relationship such as whether it is enabled, default office, ad default settings). Through multiple refactorings I have determined that both Terminal and Customer should probably be aggregate roots. This leaves me with the question of how to design my TerminalCustomerRelationship object to relate the two.
I suppose I could make the traversal from Terminal to TerminalCustomerRelationship unidirectional toward the relationship but I don't know how to break the relation from the relationship to the customer, especially since it needs to contain a reference to an Office which has a relationship to a Customer.
I'm new to this stuff and while most of DDD makes perfect sense I'm getting confused and need a fresh outlook. Can someone give me their opinion on how to deal with this situation?
Please note that I say Relationship not relation. In my current view it deserves to be an object in the same way that a Marriage would be an object in an application for a wedding chapel. Its most visible purpose is that it relates two objects, but it has other properties that rightfully belong to it as well.
By your description, you definitely need a "TerminalCustomerRelationship" entity to track the associated information. I would also convert the 'IsEnabled' flag into a first class 'Event' entity with a timestamp - this gives you the ability to save a history of the state changes (a more realistic view of what's happening in the domain.)
Here's a sample application (in VS2008) that refects your problem. You can tweak/test the code until the relationships make sense. Run "bin/debug/TerminalSampleApp.exe" and right-click "Terminal->Create Example" to get started.
Let me know if you find it useful.
Names can often clarify an object's responsibilities and bring a domain model into focus.
I am unclear what a Site is and that makes the entire model confusing, which makes it difficult for me to offer better advice. If a Site were a Vendor, for instance, then it would be easy to rename SiteCustomerRelationship as a Contract. In that context it makes perfect sense for Contract to be its own entity, and have the the model look like Vendor-Contract-Customer-Office.
There are other ways to look at this as well. Udi has a decent post on this sort of many-to-many relationship here.
You should not have a object Like SiteCustomerRelationship, its DB specific.
If its truly DDD you should have a Relation like:
Aggregate<Site> Customer.Site
IEnumerable<Aggregate<Office>> Customer.Offices
and perhaps
Aggregate<Office> Customer.DefaultOffice
I am working on car dealership business domain model/UML class diagram.
I am new to modeling, so I would like to know how to validate the class diagram. It's very important for me to have an appropriate, if not 100 percent correct, class diagram to use further development (use cases, etc.).
Is it possible to build a completely incorrect model? Or are there only appropriate and less appropriate models?
If I have a Customer associated with SalesTeam modeling a customer being served by SalesTeam, is that wrong? I have seen in examples of Customer being associated with Order, Order with ItemOrder and ItemOrder with ItemInventory. Where the SalesTeam or Staff is associated with Order.
How do I validate my model and relationships?
To validate domain models, do the following.
Write use cases. During the writing, make sure you're using nouns and verbs in a consistent way. To be sure that your nouns make sense, be sure to record notes in the domain model.
Walk through each use case, following along on your domain model. At the entities there? Relationships required for navigation? Attributes of each entity?
Since it's a domain model, try to avoid describing things as classes -- they're usually real-world entities.
For example "customer entity in direct relationship with sales team entity" is something you'll learn from the use cases. For example, customers are associated with orders, but the order is created by the sales team. So, you have two navigation paths between customer and order: direct and via the sales team. Both appear (to me) to be true.
You must compare your domain model with your use cases to be sure both agree.
The short answer is that this is not very important.
Use your domain class diagrams to keep a note of what you think is in the domain, that is all. It is not your god, and it will not hurt you to change it as you go.
Domain experts should help you to validate the domain model.
As far as validating the specific relationships, as you develop the model further and investigate the collaborations between objects you will discover more and different relationships. You will need to revisit the domain model often during your analysis and development.
I don't think it matters that it's 'correct' up front (i.e. before you move onto looking at use cases and further analysis), only that it is useful - it gives you a conceptual model of the problem and what the main classes involved are. It isn't going to be finished until the software is no longer being developed or maintained.
If it represents the way you view the problem right now, it's good enough for you to start further analysis. Revise it as your view of the problem changes and you learn more.