Are many-to-many table structures defined as Value Objects in DDD? What if my many-to-many structure has a unique id?
Also, what about 1-to-many relationships? For instance, if i have 2 structures Post & Comment with 1-to-many (respectively) wouldn't Comment be a Value Object since it technically cannot exist without a corresponding Post? But what if it has a unique Id (Comment_Id)? That makes it identifiable and therefore a non-value object entity? So, which is it?
Thanks!
You are probably do not need DDD for blog. DDD is more about business logic. You will not have enough business logic to benefit from DDD.
If you still want to use DDD, not a problem. Some guidelines:
If you want something to be referensable from outside of the business logic, it should be entity. So if you need reference to comments (otherwise you will not be able to manage them effectively) you need your comments to be an entities.
You have to maximize persistence ignorance. This means that could not expose identifiers to your business logic.
Many-to-many and one-to-many relationships are modeled with plain collections. The ORM tool should map this collections properly.
Hope this helps to avoid DDD in your situation.
Related
In DDD we model the domain using several aggregates (root + entities). One such aggregate or entity can hold a reference to another aggregate root through its id.
In axon, I see the concept of aggregates and member entities, but I do not see the notion of references to other aggregates.
What am I missing? Or is this not possible in axon?
It works the same, you can hold the reference of another aggregate throught its aggregateId (String/UUID/Whatever).
I wish I could provide more insights to you but your question is rather vague =)
I thinking about modeling aggregates, invariants, data etc. There is common advice to design aggregates to be small. I have problem a with correct splitting domain and simple CRUD.
Let's assume that we have application where we are able to create project and join to it collaborators. There are a lot of informations related with project at the stage of creating (name, description, project_aims, notes, creation date, modified date, collaborators). How to correct design aggregate where there is a rule which check that we can only add 5 collaborators. Taking into consideration that fields name, description, project_aims, notes doesn't really take part in any business rule and there is only requirements that this fields should'nt be empty (those are really invariants?) should those fields really be a part of aggregate?
Is'nt that our real Domain (aggregates, entities, value objects, policies) should hold only data which take part with protecting invariants or help making business decisions?
If so, how to (create) project described above? Should class with all that nonsignificant (from a business point of view) fields be implemented as anemic model outside the Domain and Aggregate root should just have method addCollaborator which protect quantity of collaborators? Is it good idea to save anemic class object using Dao (operates on db table) and for Domain implementation of aggregate, create Repository?
How to add first collaborator during creating project as at the beggining we create anemic class object outside Domain?
Thank you for any help and advice
Papub
"How to correct design aggregate where there is a rule which check that we can only add 5 collaborators"
Project AR most likely maintains a set of collaborators and throws whenever it's size would exceed 5.
"there is only requirements that this fields should'nt be empty (those are really invariants?)"
Yes, these may be simple rules, but are still are invariants.
"should hold only data which take part with protecting invariants or help making business decisions"
This can be done when modeling ARs with EventSourcing, where you'd only materialize the properties needed to check invariants on the AR, while having all data in the full set of events.
"implemented as anemic model outside the Domain and Aggregate root should just have method addCollaborator which protect quantity of collaborators".
You could always attempt to mix CRUD-driven anemia with rich always-valid models, but the anemic/rich model decision is usually consistent for a given Bounded Context (BC), meaning you may have CRUDDy BCs and rich domain model BCs, but rarely both strategies in the same BC.
For instance, perhaps "Project Definition" is a CRUD-driven BC while "Collaboration" isin't. Those BCs may not make any sense, but it's just to give an example.
Furthermore, don't forget DDD tactical patterns are there to allow manage the complexity, they aren't hard rules. If handling a part of your AR through services and another (where there's more meat) with rich behaviors then perhaps that's acceptable. Still, I'd probably prefer CRUDDy behaviors on the ARs themselves like an update method rather than giving up in the anemic direction.
I am developing an online job portal using DDD patterns.
There are many "objects" that i have figured out like Users, Jobs, Roles, Expertise, ExperienceRange, Country, State, City, Address, Subscriptions, etc
My question is how do i figure out which of these is an entity or a value object or an aggregate? Please advice me if you have ever faced the same dilemma.
I have made the following decision:
Entities - User, Job, SubscriptionPackage
ValueObjects - Role, Expertise, ExperienceRange, City, State, Country
I know that we should not think about persistence while doing DDD modelling but a doubt has surfaced that whichever value objects i am storing in database should have an id or not?
if they have an id do they not violate the fundamental principal of ValueObjects and if we do not save them with ids then how to reference them in foreign key fields?
Please help me answer these queries.
If you can suggest which of the above mentioned objects are entities, which are value objects and which are aggregates that would be great.
Thanking in advance
When thinking of DDD, leave the DB mapping to a later stage. I know I'm repeating what you said, but just because it's true. A value object might have a DB id for other reasons (normalisation, reporting , etc).
First come up with your object model and then figure out how to map it. In some (rare) cases you might need to change slightly your object model if there's something that is too expensive to map properly (I cannot think of an example, but I don't want to be extremist).
So once more, forget about the DB - think about objects. For what reason does an entity have an id? I would say so then later it can be retrieved and modified, while keeping the same id.
And if it is a VO is because the identity is implicit in the values of the object. Does it make sense for a User to have an id? What about an Address? Or a City?... It depends.
To give the example of a city value object, if you need to map that as FK to 'cities' table, then your City object will probably have an id, but the id is not exposed. It's a detail of the implementation. While the user id would be exposed. For example a city might be linked to a province/state and that to a country.
But in another application, where users can add cities and information about them, the city might be an Entity or even an Aggregate. It really depends on your requirements.
Having said that, the list of Entities and VOs you provided looks ok in a general way, but I don't know your requirements.
To answer the first question: you can read Entities, Value Objects, Aggregates and Roots as there are some rules about what is a VO, Entity or Aggregate. The difficulty comes from how to apply them, and experience is the solution to that.
As a summary:
Entities
Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.
Value Objects
Many objects have no conceptual identity. These objects describe characteristics of a thing.
Aggregates
Aggregates draw a boundary around one or more Entities. An Aggregate enforces invariants for all its Entities for any operation it supports.
Everything I have read thus far on DDD implies only entities which encapsulate other entities are root aggregates.
What about in situations like:
WorkOrder
- idManufacturer
- WONumber
- Description
Manufacturer
- idSelf
- Name
WorkOrder references Manufacturer but would not be a child of WorkOrder as other entities might reference WorkOrder, in this case I would consider both Root entities, but the Manufacturer is not an aggregate...
Is this correct?
I once had a lightbulb moment with DDD when someone told me that entities with no children can be though of as aggregate roots.
Particularly when someone says "persist only your aggregate roots".
In your example, your aggregate roots are WorkOrder and Manufacturer. You'd have a repository for WorkOrder and one for Manufacturer.
In fact, you will mostly have aggregates with value objects only. ARs with child entites are rare. Read red book (Implementing DDD Vaughn Vernon), there is described rule of small aggregates.
The job of an aggregate root is to encapsulate and enforce invariants. It may consist of other objects but they are all interacted with through the AR. The important thing to remember about an aggregate, is that is should be independent of your chosen persistence mechanism. The majority of your aggregates should have no dependencies at all!
I may be mistaken but it looks like the idManufacturer is a foreign key. This would suggest (not always the case) it is not encapsulated. The thing that took me a while to get my head around was that the fields within an aggregate are private. This raises the question of how you save it's state and how you get data to put on the UI. There are lots of ways to that. My preferred approach is to use Event Sourcing and CQRS. I can then build out UI's from the events that my aggregates produce. I can also persist those events and use them to rebuild my aggregate.
I've gone into a lot more depth on my blog, you may want to take a look at How to Build an Aggregate Root! . You may also find helpful a post on building a master details screen when using CQRS and Event sourcing, that can be found here!
I hope that helps.
I am into my first week of DDD and have a couple of entities with aggregate roots defined.
I read that no external entity outside of an aggregate can reference an entity in an aggregate, so the external entity has to reference the aggregate root.
Well, unless I have modelled my solution incorrectly I need a reference to an entity behind the aggregate root. How do I handle this situation or do I have to remodel my domain to avoid this situation?
JD
You probably could refine your model. If an external reference to an entity inside your aggregate is required, then that is a strong indicator that the internal entity might be an aggregate root itself.
This of course is general advice since I don't know anything about your specific model.
For great advice concerning aggregate design, have a look at this paper by Vaughn Vernon. In Part I, "Modeling of an Aggregate", he specifically addresses aggregate granularity which I found very enlightening.