I just want to check really quickly. Say I have two entities in a data model: Catalog, and Product. They have a many-to-many relationship with each other, and both are required (a Catalog must have at least one Product, and all Products must each belong to at least one Catalog). So if I was to delete a Product, its deletion should be Nullify, of course.
But what should the deletion policy be for Catalog? If a Catalog is deleted, not all of its Products necessarily exclusively belong to it. A Product may belong to more than one Catalog. So I definitely shouldn't use Cascade. However, is Nullify sufficient? What if I end up with dangling Products that don't belong to a Catalog? What does Core Data have built in that would resolve this issue with many-to-many schemas? Do I need to modify my schema?
Nullify is sufficient, and many-to-many sounds right. The specific constraint you want (deleting orphans) is not directly enforceable by core data, though, so you get to do a little cleanup yourself.
Specifically, implement willSave in your entity classes, and have each entity test: am I not deleted; and, do I have no associated (products/catalogs)? If so, delete myself. (the not-deleted test is important to avoid an infinite loop of willSaves.)
This postpones the deletion of the orphaned catalogs or products until save time. This is probably not a problem.
I've implemented rgeorge's answer, and thought the exact code might be helpful to other people:
- (void)willSave
{
[super willSave];
if (self.isDeleted)
return;
if (self.products.count == 0)
[self.managedObjectContext deleteObject:self];
}
Swift translation of Andy and JosephH
override func willSave() {
super.willSave()
if self.deleted {
return
}
if self.products.count == 0 {
self.managedObjectContext?.deleteObject(self)
}
}
Related
I'm new to DDD. Now I've an aggregate Team and entities TeamMember:
class Team {
members: Map<TeamMemberId, TeamMember>;
add(member) {
assert(!members.has(member.id), "Team Member is already exists");
this.members.set(member.id, member);
}
}
When I execute AddTeamMemberCommand, the repository will load the entire aggregate from MongoDB.
when the team is large, this may seem unacceptable.
I found the following from google and stackoverflow:
Use Id references instead of entities
Lazy load
Redesign aggregation
....
I'm not sure which solution is right for me, or is there a better, more generic solution for this scenario? Is there a GitHub sample project i can look at?
Thank you very much.
is there a better, more generic solution for this scenario?
For reads/queries, where you aren't going to be changing the aggregate at all, lazy loading is fine. In the world of CQRS, we might even avoid loading the aggregate altogether, instead just fetching a read-only copy of the information that we need.
An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes.
If we're trying to make a change to an aggregate, and are tempted to leave a bunch of unnecessary information unloaded, that may imply that our aggregate boundaries are in the wrong places, and that we should be re-designing our domain model so that the loaded information better fits with what we need.
For example, if you are trying to update just Bob, not the entire team, then that may be a hint that Bob isn't an entity inside of the Team aggregate, but is instead belongs in a different, smaller aggregate, which has some relationship with team.
Mauro Servienti's talk on aggregate boundaries may be a good starting point.
[ Follow up from this question & comments: Should entity have methods and if so how to prevent them from being called outside aggregate ]
As the title says: i am not clear about what is the actual/precise purpose of entity as a child in aggregate?
According to what i've read on many places, these are the properties of entity that is a child of aggregate:
It has identity local to aggregate
It cannot be accessed directly but through aggregate root only
It should have methods
It should not be exposed from aggregate
In my mind, that translates to several problems:
Entity should be private to aggregate
We need a read only copy Value-Object to expose information from an entity (at least for a repository to be able to read it in order to save to db, for example)
Methods that we have on entity are duplicated on Aggregate (or, vice versa, methods we have to have on Aggregate that handle entity are duplicated on entity)
So, why do we have an entity at all instead of Value Objects only? It seams much more convenient to have only value objects, all methods on aggregate and expose value objects (which we already do copying entity infos).
PS.
I would like to focus to child entity on aggregate, not collections of entities.
[UPDATE in response to Constantin Galbenu answer & comments]
So, effectively, you would have something like this?
public class Aggregate {
...
private _someNestedEntity;
public SomeNestedEntityImmutableState EntityState {
get {
return this._someNestedEntity.getState();
}
}
public ChangeSomethingOnNestedEntity(params) {
this._someNestedEntity.someCommandMethod(params);
}
}
You are thinking about data. Stop that. :) Entities and value objects are not data. They are objects that you can use to model your problem domain. Entities and Value Objects are just a classification of things that naturally arise if you just model a problem.
Entity should be private to aggregate
Yes. Furthermore all state in an object should be private and inaccessible from the outside.
We need a read only copy Value-Object to expose information from an entity (at least for a repository to be able to read it in order to save to db, for example)
No. We don't expose information that is already available. If the information is already available, that means somebody is already responsible for it. So contact that object to do things for you, you don't need the data! This is essentially what the Law of Demeter tells us.
"Repositories" as often implemented do need access to the data, you're right. They are a bad pattern. They are often coupled with ORM, which is even worse in this context, because you lose all control over your data.
Methods that we have on entity are duplicated on Aggregate (or, vice versa, methods we have to have on Aggregate that handle entity are duplicated on entity)
The trick is, you don't have to. Every object (class) you create is there for a reason. As described previously to create an additional abstraction, model a part of the domain. If you do that, an "aggregate" object, that exist on a higher level of abstraction will never want to offer the same methods as objects below. That would mean that there is no abstraction whatsoever.
This use-case only arises when creating data-oriented objects that do little else than holding data. Obviously you would wonder how you could do anything with these if you can't get the data out. It is however a good indicator that your design is not yet complete.
Entity should be private to aggregate
Yes. And I do not think it is a problem. Continue reading to understand why.
We need a read only copy Value-Object to expose information from an entity (at least for a repository to be able to read it in order to
save to db, for example)
No. Make your aggregates return the data that needs to be persisted and/or need to be raised in a event on every method of the aggregate.
Raw example. Real world would need more finegrained response and maybe performMove function need to use the output of game.performMove to build propper structures for persistence and eventPublisher:
public void performMove(String gameId, String playerId, Move move) {
Game game = this.gameRepository.load(gameId); //Game is the AR
List<event> events = game.performMove(playerId, move); //Do something
persistence.apply(events) //events contains ID's of entities so the persistence is able to apply the event and save changes usign the ID's and changed data wich comes in the event too.
this.eventPublisher.publish(events); //notify that something happens to the rest of the system
}
Do the same with inner entities. Let the entity return the data that changed because its method call, including its ID, capture this data in the AR and build propper output for persistence and eventPublisher. This way you do not need even to expose public readonly property with entity ID to the AR and the AR neither about its internal data to the application service. This is the way to get rid of Getter/Setters bag objects.
Methods that we have on entity are duplicated on Aggregate (or, vice versa, methods we have to have on Aggregate that handle entity
are duplicated on entity)
Sometimes the business rules, to check and apply, belongs exclusively to one entity and its internal state and AR just act as gateway. It is Ok but if you find this patter too much then it is a sign about wrong AR design. Maybe the inner entity should be the AR instead a inner entity, maybe you need to split the AR into serveral AR's (inand one the them is the old ner entity), etc... Do not be affraid about having classes that just have one or two methods.
In response of dee zg comments:
What does persistance.apply(events) precisely do? does it save whole
aggregate or entities only?
Neither. Aggregates and entities are domain concepts, not persistence concepts; you can have document store, column store, relational, etc that does not need to match 1 to 1 your domain concepts. You do not read Aggregates and entities from persitence; you build aggregates and entities in memory with data readed from persistence. The aggregate itself does not need to be persisted, this is just a possible implementation detail. Remember that the aggregate is just a construct to organize business rules, it's not a meant to be a representation of state.
Your events have context (user intents) and the data that have been changed (along with the ID's needed to identify things in persistence) so it is incredible easy to write an apply function in the persistence layer that knows, i.e. what sql instruction in case of relational DB, what to execute in order to apply the event and persist the changes.
Could you please provide example when&why its better (or even
inevitable?) to use child entity instead of separate AR referenced by
its Id as value object?
Why do you design and model a class with state and behaviour?
To abstract, encapsulate, reuse, etc. Basic SOLID design. If the entity has everything needed to ensure domain rules and invariants for a operation then the entity is the AR for that operation. If you need extra domain rules checkings that can not be done by the entity (i.e. the entity does not have enough inner state to accomplish the check or does not naturaly fit into the entity and what represents) then you have to redesign; some times could be to model an aggregate that does the extra domain rules checkings and delegate the other domain rules checking to the inner entity, some times could be change the entity to include the new things. It is too domain context dependant so I can not say that there is a fixed redesign strategy.
Keep in mind that you do not model aggregates and entities in your code. You model just classes with behaviour to check domain rules and the state needed to do that checkings and response whith the changes. These classes can act as aggregates or entities for different operations. These terms are used just to help to comunicate and understand the role of the class on each operation context. Of course, you can be in the situation that the operation does not fit into a entity and you could model an aggregate with a V.O. persistence ID and it is OK (sadly, in DDD, without knowing domain context almost everything is OK by default).
Do you wanna some more enlightment from someone that explains things much better than me? (not being native english speaker is a handicap for theese complex issues) Take a look here:
https://blog.sapiensworks.com/post/2016/07/14/DDD-Aggregate-Decoded-1
http://blog.sapiensworks.com/post/2016/07/14/DDD-Aggregate-Decoded-2
http://blog.sapiensworks.com/post/2016/07/14/DDD-Aggregate-Decoded-3
It has identity local to aggregate
In a logical sense, probably, but concretely implementing this with the persistence means we have is often unnecessarily complex.
We need a read only copy Value-Object to expose information from an
entity (at least for a repository to be able to read it in order to
save to db, for example)
Not necessarily, you could have read-only entities for instance.
The repository part of the problem was already addressed in another question. Reads aren't an issue, and there are multiple techniques to prevent write access from the outside world but still allow the persistence layer to populate an entity directly or indirectly.
So, why do we have an entity at all instead of Value Objects only?
You might be somewhat hastily putting concerns in the same basket which really are slightly different
Encapsulation of operations
Aggregate level invariant enforcement
Read access
Write access
Entity or VO data integrity
Just because Value Objects are best made immutable and don't enforce aggregate-level invariants (they do enforce their own data integrity though) doesn't mean Entities can't have a fine-tuned combination of some of the same characteristics.
These questions that you have do not exist in a CQRS architecture, where the Write model (the Aggregate) is different from a Read model. In a flat architecture, the Aggregate must expose read/query methods, otherwise it would be pointless.
Entity should be private to aggregate
Yes, in this way you are clearly expressing the fact that they are not for external use.
We need a read only copy Value-Object to expose information from an entity (at least for a repository to be able to read it in order to save to db, for example)
The Repositories are a special case and should not be see in the same way as Application/Presentation code. They could be part of the same package/module, in other words they should be able to access the nested entities.
The entities can be viewed/implemented as object with an immutable ID and a Value object representing its state, something like this (in pseudocode):
class SomeNestedEntity
{
private readonly ID;
private SomeNestedEntityImmutableState state;
public getState(){ return state; }
public someCommandMethod(){ state = state.mutateSomehow(); }
}
So you see? You could safely return the state of the nested entity, as it is immutable. There would be some problem with the Law of Demeter but this is a decision that you would have to make; if you break it by returning the state you make the code simpler to write for the first time but the coupling increases.
Methods that we have on entity are duplicated on Aggregate (or, vice versa, methods we have to have on Aggregate that handle entity are duplicated on entity)
Yes, this protect the Aggregate's encapsulation and also permits the Aggregate to protect it's invariants.
I won't write too much. Just an example. A car and a gear. The car is the aggregate root. The gear is a child entity
I am building a system to manage person information. I have an ever growing aggregate root called Person. It now has hundreds of related objects, name, addresses, skills, absences, etc. My concern is that the Person AR is both breaking SRP and will create performance problems as more and more things (esp collections) get added to it.
I cannot see how with DDD to break this down into smaller objects. Taking the example of Absences. The Person has a collection of absence records (startdate, enddate, reason). These are currently managed through the Person (BookAbsence, ChangeAbsence, CancelAbsence). When adding absences I need to validate against all other absences, so I need an object which has access to the other absences in order to do this validation.
Am I missing something here? Is there another AR I have not identified? In the past I would have done this via an "AbsenceManager" service, but would like to do it using DDD.
I am fairly new to DDD, so maybe I am missing something.
Many Thanks....
The Absence chould be modeled as an aggregate. An AbsenceFactory is reposible for validating against other Absence s when you want to add a new Absence.
Code example:
public class AbsenceFactory {
private AbsenceRepository absenceRepository;
public Absence newAbsenceOf(Person person) {
List<Absence> current =
absenceRepository.findAll(person.getIdentifier());
//validate and return
}
}
You can find this pattern in the blue book (section 6.2 Factory if I'm not mistaken)
In other "modify" cases, you could introduce a Specification
public class SomeAbsenceSpecification {
private AbsenceRepository absenceRepository;
public SomeAbsenceSpecification(AbsenceRepository absenceRepository) {
this.absenceRepository=absenceRepository;
}
public boolean isSatisfiedBy(Absence absence) {
List<Absence> current =
absenceRepository.findAll(absence.getPersonIdentifier());
//validate and return
}
}
You can find this pattern in the blue book(section 9.2.3 Specification)
This is indeed what makes aggregate design so tricky. Ownership does not necessarily mean aggregation. One needs to understand the domain to be able to give a proper answer so we'll go with the good ol' Order example. A Customer would not have a collection of Order objects. The simplest rule is to think about deleting an AR. Those objects that could make sense in the absence of the AR probably do not belong on the AR. A Customer may very well have a collection of ActiveOrder objects, though. Of course there would be an invariant stating that a customer cannot be deleted if it has active orders.
Another thing to look out for is a bloated bounded context. It is conceivable that you could have one or more bounded contexts that have not been identified leading to a situation where you have an AR doing too much.
So in your case you may very well still be interested in the Absence should the Customer be deleted. In the case of an OrderLine it has no meaning without its Order. So no lifecycle of its own.
Hope that helps ever so slightly.
I am building a system to manage person information.
Are you sure that a simple CRUD application that edit/query RDBMS's tables via SQL, wouldn't be a cheaper approach?
If you can express the most of the business rules in term of data relations and table operations, you shouln't use DDD at all.
I have an ever growing aggregate root called Person.
If you actually have complex business rules, an ever growing aggregate is often a syntom of undefined (or wrongly defined) context boundaries.
I have the need to store arbitrary references to entities within my Raven Database. Sometimes the entity is an aggregate root (see Events below) and other times it is a value entity (see Sessions below). I'm currently planning to store the references as Lucene queries (or a Lucene-like syntax.) Has anyone done anything like this? Am I heading down a difficult path?
Some of my concerns are:
Value entities are unlikely to provide identifiers, can I expect to reliably reference value entities?
Individual entities should be unaware of (decoupled from) the Arbitrary Relationship infrastructure, what is the best way to infer the queries from complex object graphs?
Limiting the relationships to only Aggregate roots (and preventing references to value entities) would simplify the problem, but it would require me to restructure my Event/Session documents. I'd like these two systems to remain decoupled (concerns of one should not impact the other.)
I've included example documents below to illustrate my scenario. Any thoughts, ideas, guidance, or examples would be very appreciated.
Events Collection
{
Id: “30f6...54a7”,
Title: “Annual Meeting”
Sessions: [
{
Code: “COM001”,
Title: “Opening Ceremony”
},
{
Code: “TEC201”,
Title: “Intermediate Tech”
}
]
}
People Collection
{
Id: "45a8...f209",
Name: "Chad"
}
Arbitrary Relationships Collection
{
Id: “b613...8ebb”,
SubjectEntityQuery: "People.Id:45a8...f209",
TargetEntityQuery: “Events.Id:30f6...54a7.Sessions,Code:COM001”,
Action: "Attended Session",
Story: "Chad attended the Opening Ceremony session"
}
Edit
I'd like to give more detail on the arbitrary relationships. We will have the ability to extend the system to respond to system events and record the interaction between two entities. We have many more entities than Events, Sessions, and People. The relationship may be a person sharing a link or a tweet about a hashtag. Effectively, the Arbitrary Relationships collection becomes a graph-like structure that allows us to see all ~interactions~ for a given entity.
This is screaming relational design.
The easiest way to do this is make the Relationship an object and its Subject and Target fields an array of strings holding IDs of the actual documents it references. This way you can take advantage of Includes to load them along with the relationship document. Eitherway, I don't see how storing a Lucene query syntax helps here.
There may be a better way to model this, but it really depends on your business model and on the thing you are trying to achieve.
Also, you might want to get rid of GUID IDs, just use Raven's conventions.
I'm one of many trying to understand the concept of aggregate roots, and I think that I've got it!
However, when I started modeling this sample project, I quickly ran into a dilemma.
I have the two entities ProcessType and Process. A Process cannot exist without a ProcessType, and a ProcessType has many Processes. So a process holds a reference to a type, and cannot exist without it.
So should ProcessType be an aggregate root? New processes would be created by calling processType.AddProcess(new Process());
However, I have other entities that only holds a reference to the Process, and accesses its type through Process.Type. In this case it makes no sense going through ProcessType first.
But AFAIK entities outside the aggregate are only allowed to hold references to the root of the aggregate, and not entities inside the aggregate. So do I have two aggregates here, each with their own repository?
I largely agree with what Sisyphus has said, particularly the bit about not constricting yourself to the 'rules' of DDD that may lead to a pretty illogical solution.
In terms of your problem, I have come across the situation many times, and I would term 'ProcessType' as a lookup. Lookups are objects that 'define', and have no references to other entities; in DDD terminology, they are value objects. Other examples of what I would term a lookup may be a team member's 'RoleType', which could be a tester, developer, project manager for example. Even a person's 'Title' I would define as a lookup - Mr, Miss, Mrs, Dr.
I would model your process aggregate as:
public class Process
{
public ProcessType { get; }
}
As you say, these type of objects typically need to populate dropdowns in the UI and therefore need their own data access mechanism. However, I have personally NOT created 'repositories' as such for them, but rather a 'LookupService'. This for me retains the elegance of DDD by keeping 'repositories' strictly for aggregate roots.
Here is an example of a command handler on my app server and how I have implemented this:
Team Member Aggregate:
public class TeamMember : Person
{
public Guid TeamMemberID
{
get { return _teamMemberID; }
}
public TeamMemberRoleType RoleType
{
get { return _roleType; }
}
public IEnumerable<AvailabilityPeriod> Availability
{
get { return _availability.AsReadOnly(); }
}
}
Command Handler:
public void CreateTeamMember(CreateTeamMemberCommand command)
{
TeamMemberRoleType role = _lookupService.GetLookupItem<TeamMemberRoleType>(command.RoleTypeID);
TeamMember member = TeamMemberFactory.CreateTeamMember(command.TeamMemberID,
role,
command.DateOfBirth,
command.FirstName,
command.Surname);
using (IUnitOfWork unitOfWork = UnitOfWorkFactory.CreateUnitOfWork())
_teamMemberRepository.Save(member);
}
The client can also make use of the LookupService to populate dropdown's etc:
ILookup<TeamMemberRoleType> roles = _lookupService.GetLookup<TeamMemberRoleType>();
Not so simple. ProcessType is most likley a knowledge layer object - it defines a certain process. Process on the other hand is an instance of a process that is ProcessType. You probably really don't need or want the bidirectional relationship. Process is probably not a logical child of a ProcessType. They typically belong to something else, like a Product, or Factory or Sequence.
Also by definition when you delete an aggregate root you delete all members of the aggregate. When you delete a Process I seriously doubt you really want to delete ProcessType. If you deleted ProcessType you might want to delete all Processes of that type, but that relationship is already not ideal and chances are you will not be deleting definition objects ever as soon as you have a historical Process that is defined by ProcessType.
I would remove the Processes collection from ProcessType and find a more suitable parent if one exists. I would keep the ProcessType as a member of Process since it probably defines Process. Operational layer (Process) and Knowledge Layer(ProcessType) objects rarely work as a single aggregate so I would have either Process be an aggregate root or possibly find an aggregate root that is a parent for process. Then ProcessType would be a external class. Process.Type is most likely redundant since you already have Process.ProcessType. Just get rid of that.
I have a similar model for healthcare. There is Procedure (Operational layer) and ProcedureType (knowledge layer). ProcedureType is a standalone class. Procedure is a child of a third object Encounter. Encounter is the aggregate root for Procedure. Procedure has a reference to ProcedureType but it is one way. ProcedureType is a definition object it does not contain a Procedures collection.
EDIT (because comments are so limited)
One thing to keep in mind through all of this. Many are DDD purists and adamant about rules. However if you read Evans carefully he constantly raises the possibility that tradeoffs are often required. He also goes to pretty great lengths to characterize logical and carefully thought out design decisions versus things like teams that do not understand the objectives or circumvent things like aggregates for the sake of convenience.
The important things is to understand and apply the concepts as opposed to the rules. I see many DDD that shoehorn an application into illogical and confusing aggregates etc for no other reason than because a literal rule about repositories or traversal is being applied, That is not the intent of DDD but it is often the product of the overly dogmatic approach many take.
So what are the key concepts here:
Aggregates provide a means to make a complex system more manageable by reducing the behaviors of many objects into higher level behaviors of the key players.
Aggregates provide a means to ensure that objects are created in a logical and always valid condition that also preserves a logical unit of work across updates and deletes.
Let's consider the last point. In many conventional applications someone creates a set of objects that are not fully populated because they only need to update or use a few properties. The next developer comes along and he needs these objects too, and someone has already made a set somewhere in the neighborhood fora different purpose. Now this developer decides to just use those, but he then discovers they don't have all the properties he needs. So he adds another query and fills out a few more properties. Eventually because the team does not adhere to OOP because they take the common attitude that OOP is "inefficient and impractical for the real world and causes performance issues such as creating full objects to update a single property". What they end up with is an application full of embedded SQL code and objects that essentially randomly materialize anywhere. Even worse these objects are bastardized invalid proxies. A Process appears to be a Process but it is not, it is partially populated in different ways any given point depending on what was needed. You end up with a ball mud of numerous queries to continuously partially populate objects to varying degrees and often a lot of extraneous crap like null checks that should not exist but are required because the object is never truly valid etc.
Aggregate rules prevent this by ensuring objects are created only at certain logical points and always with a full set of valid relationships and conditions. So now that we fully understand exactly what aggregate rules are for and what they protect us from, we also want to understand that we also do not want to misuse these rules and create strange aggregates that do not reflect what our application is really about simply because these aggregate rules exists and must be followed at all times.
So when Evans says create Repositories only for aggregates he is saying create aggregates in a valid state and keep them that way instead of bypassing the aggregate for internal objects directly. You have a Process as a root aggregate so you create a repository. ProcessType is not part of that aggregate. What do you do? Well if an object is by itself and it is an entity, it is an aggregate of 1. You create a repository for it.
Now the purist will come along and say you should not have that repository because ProcessType is a value object, not an entity. Therefore ProcessType is not an aggregate at all, and therefore you do not create a repository for it. So what do you do? What you don't do is shoehorn ProcessType into some kind of artificial model for no other reason than you need to get it so you need a repository but to have a repository you have to have an entity as an aggregate root. What you do is carefully consider the concepts. If someone tells you that repository is wrong, but you know that you need it and whatever they may say it is, your repository system is valid and preserves the key concepts, you keep the repository as is instead of warping your model to satisfy dogma.
Now in this case assuming I am correct about what ProcessType is, as the other commentor noted it is in fact a Value Object. You say it cannot be a Value Object. That could be for several reasons. Maybe you say that because you use NHibernate for example, but the NHibernate model for implementing value objects in the same table as another object does not work. So your ProcessType requires an identity column and field. Often because of database considerations the only practical implementation is to have value objects with ids in their own table. Or maybe you say that because each Process points to a single ProcessType by reference.
It does not matter. It is a value Object because of the concept. If you have 10 Process objects that are of the same ProcessType you have 10 Process.ProcessType members and values. Whether each Process.ProcessType points to a single reference, or each got a copy, they should still by definition all be exactly the same things and all be completely interchangeable with any of the other 10. THAT is what makes it a value Object. The person who says "It has an Id therefore is cannot be a value Object you have an entity" is making a dogmatic error. Don't make the same error, if you need an ID field give it one, but don't say "it can't be a Value Object" when it in fact is albeit one that for other reason you had to give an Id to.
So how do you get this one right and wrong? ProcessType is a Value Object, but for some reason you need it to have an Id. The Id per se does not violate the rules. You get it right by having 10 processes that all have a ProcessType that is exactly the same. Maybe each has a local deeep copy, maybe they all point to one object. but each is identical either way, ergo each has an Id = 2, for example. You get is wrong when you do this: 10 Processes each have a ProcessType, and this ProcessType is identical and completely interchangeable EXCEPT now each also has it's own unique Id as well. Now you have 10 instances of the same thing but they vary only in Id, and will always vary only in Id. Now you no longer have a Value Object, not because you gave it an Id, but because you gave it an Id with an implementation that reflects the nature of an entity - each instance is unique and different
Make sense?
Look i think you have to restructure your model. Use ProcessType like a Value Object and Process Agg Root.
This way Every Process has a processType
Public class Process
{
Public Process()
{
}
public ProcessType { get; }
}
for this u just need 1 agg root not 2.