As i read through software engineering appendix 1 from Roger Pressman book that
an association between two classes means that there is a structural
relationship between them
what structural relationship means?
UML differentiates 'structural' and 'behavioural' models. Class Diagrams, Package Diagrams and a few other capture the structural aspects. State/Sequence/Activity Diagrams capture behavioural aspects.
'Structural' means it holds over time. For example, the association between Order and OrderLines ("Order consists of 1 or more OrderLines / OrderLine is part of exactly one Order"). Or Dog and Person ("Dog is owned by exactly one Person / Person owns many Dogs"). Used well, Associations capture invariant rules from the problem domain. To use the Dog example: the association says a Dog can't ever be owned by more than one Person at any given time. Doesn't matter if the Dog is running, sitting, or eating: it must have exactly one Owner. Note also the owner could change over time: but there can never be more than one at any point.
An alternative is to think of Associations as the kind of thing that might be captured using foreign keys in a relational database.
hth.
Related
I have an assignment for school where I'm asked to represent the system of a company that I am to upgrade with a domain model and draw a conceptual class diagram with the four most important use cases of the system. I don't really understand the difference between the two, can someone help me ?
In short
Domain model and a conceptual mean different things to different people. There is no universal authoritative definition of these terms.
Nevertheless, objectively a domain is more than interrelated classes. If we consider that conceptual means independent of any solution implementation, we can claim that a conceptual class diagram is a subpart of the domain model.
Some more arguments
A domain model describes the elements of the real word for which a software shall provide a solution.
For example, for a real estate application:
you’d have “business objects” such as real estate assets (houses, flats, …), owners, tenants, sellers, buyers, agents, contracts, payments, geographical regions, etc.
But you would also have domain logic, such as the lifecycle of things: at first a party can be a prospect for an asset, then an interested prospect after a visit, then a tenderer if a bid is submitted, the. a buyer if the bid is accepted. The domain model can also describe business rules, e.g. if a tenderer proposes a price below the price demanded by the seller, the agent has to insure agreement of the seller before continuing negotiations.
DDD practitioners would also remind us that domain entities and aggregates (the things) are related to domain events that express what happens to entities and aggregates.
Hence, a domain is more than interrelated classes. If you’re bot convinced, imagine a Model-View-Controller application where the Model would ignore the business logic: would it be useful?
The term “conceptual” means something abstract that is independent of any concrete implementation. In this regard, a conceptual class diagram refers in principle to a diagram of classes that describes the domain, independently of any concrete/implementable solution.
As a consequence, a conceptual class diagram in UML only describes a static subpart of the domain model. Because by construction, the class diagram is designed for representing the static structure of classes. UML foresees other diagrams to describe the dynamic aspects of a system or its domain, such as activity diagrams, sequence diagrams or state diagrams, that allow to focus on some dynamic parts of the domain.
So a conceptual class diagram can only be a part of the domain model.
You’ll nevertheless find articles and peers who use the term “domain model” to refer to the “model of the entities of a domain”. This is a misleading shortcut in the language.
We've been given an assignment in which we are to create a conceptual model, described by a text document. There are a number of constraints given in the document, but we have also been instructed not to use constraints in the model.
We have been able to work around a few constraints, but there is one that we've been unable to tackle. I've made up a scenario that is somewhat similar to the part of the assignment that we're having issues with.
You've been tasked to create a model of the structure of a game studio. The company consists of a number of departments, and each department has at least one employee. Each employee works at a single department. There are three different types of employees: developers, designers and engineers.
In addition to this, there are a number of leadership roles that employees can have: Head of Department, Deputy Head of Department, CTO or CEO (Yes, CTO and CEO are roles that regular employees have). Each department must have 1 Head of Department and at least one Deputy Head of Department.
In addition to this, there can only be one CTO and one CEO, and these roles can only be held by engineers. Each employee can only have a single leadership role.
To solve this, we've made up an additional, abstract entity: BasicRole. This entity is a specialisation of LeadershipRole, and is a generalisation of the three roles that any employee can hold. That solves one of the problems, and now we can simply create appropriate associations between Designer/Developer and BasicRole
However, we also want Engineer to have an association with BasicRole in addition to associations to CEO and CTO. Adding those associations results in a conceptual model that looks as such:
However, this is problematic because now we're saying that an engineer can have anywhere between 0 and 3 roles.
We've considered including Company as an entity and adding associations between Company and CTO/CEO, to specify that way that the company can only have one of each, but we've been told over and over during this course not to include the thing that we're modeling as an entity in the model.
Now, it seems as if all our problems could be solved with constraints (if we were to go ahead and read up on those), with some sort of xor for the three associations. However, seeing as we've been instructed not to use constraints in the conceptual model, we're at a loss.
If you associate your Engineer to LeadershipRole (with multiplicity 0..1) removing your two relationships from Engineer to CTO, CEO and LowerRole you will get the expected result:
Each employee can only have a single leadership role.
Since LeadershipRole is abstract it has to be either CEO, CTO, HeadOfDepartment or DeputyHOD) but due to the multiplicity can't be more than one at the same time.
The "we've been told over and over during this course not to include the thing that we're modelling as an entity in the model" statement is correct if you're designing the code-level documentation but it is normal to put the entity representing the whole organisation you're modelling. In other words - don't put System (or however you call your system) in your system's model. But Company is something you model within your system.
2 options for "Each department must have 1 Head of Department and at least one Deputy Head of Department."
Redefinition
Nested notation
2 options for "these roles can only be held by engineers"
Redefinition
Generalization
Total 4=2*2 options
Hi I am new to graph database modeling and have some doubts about expressing an endorsment for a service provided by a Person. The use case is the following. PersonA gives Endorsement to a Service provided by PersonB.
The key point is that If I am recipient of the endorsment, I would like to know who has endorsed me. I have come up with several scenarios on how I could potentialy do that, but because of my lack of experience I have doubts on what would be the best aproach.
Scenario 1.
Endorsment is expressed direcly as a relationship and the service falls as a property under the endorsment So it will look like:
PersonA-------ENDORSE{service}--->PersonB
Scenario 2
I model an entity named Service. The problem is that when I do the relationship "ENDORSE" to service I would loose information on who am I endorsing. So I would have to keep a property in the relationship on who am I endorsing. Then the PersonB would AQUIRE endorsment for the SERVICE but he would not know who has actualy givern the endorsment. So..... it will look like this:
PERSONA----ENDORSE{personB}--->Service------ENDORSMENT{personA}--->PERSONB
Does this make sense ?
Scenario 3:
I normalize the second relationship "ENDORSMENT" and exclude the personA as a property , but than I need to query all Person to find out who have they endorsed.
How would you model this kind of relationship ?
Two important principles for validating a data model for a graph database:
if an entity or fact can be used more than once, then it should be stored
as the node
if the relationship of two nodes requires to store node
identifiers, then this relationship must be transformed into a node
So #Raj pointed the right way, in which case the model might look like this:
I recommend you read this:
https://neo4j.com/graph-databases-book/
http://patterns.dataincubator.org/book/
The second approach looks good, you don't have to add these properties on relationships.
It's possible to get person A who endorsed person B for service S.
The only issue with this is there will be multiple nodes for any service S. If that's not acceptable.
You can replace the Service node in the second approach with Endorse node E and connect this E to service node S.
So there will be four types of nodes.
EDIT:
Adding an image for clarification.
Rename REL1 and REL2 as you wish.
#Stdob suggested some good names for these relationships.
I am learning UML and I've focused on a Netflix-like project on which to practice on.
I've put together a class association diagram, but have been told that the multiplicities are incorrect. The multiplicities in red represent what I think they should be changed to.
Could someone please offer some clarity of where I have gone wrong?
Please, ask any questions that would help gain a fuller understanding of the diagram in reference to the project.
Thanks in advance.
Movie Catalog - User: The 0..* multiplicity is correct. There can be an arbitrary number of users and not only a single one. That would only work if that Netflix was made for a very single person. And that would be pointless, I guess.
Movie - Membership - Member: This represents an association class for the user to track payments (and access, etc.). So it must be the 1, not 0..*. The stream is sent for that single Membership where it is accounted. An association class has a n-1-1-m relation.
In your model it looks like this:
And from my POV it is:
since the Membership is just used to control access to the movies. It is arguable whether there is only a 1-1 relation and not a (preferable) m-1 relation between Member and Membership.
I am about to develop a solution for a customer where the basic entity is a member and members can have different multiple social relationships with other members. For instance Lets say we have four types of members Doctors, Specialist, Nurses and Patients. So one or more Doctors can consult one or more Specialists, One or more Doctors can treat one or more Patients. One or more Doctor is in Charge of one or more Nurses. So if I were to use a Relational DB a high degree of recursion would be necessary (as All entities must be members). Whereas recursion is not necessary in a Graph data model.
Is it then safe to say it is better to use a Graph database for a social type application where members may have different or overlapping roles.
A graph database would be good at modelling these kinds of relationships. There's a few ways that you might model it. You could think of a vertex as being a Member with different edges from Member to other Members representing the types of relationships:
Member --consults--> Member (physician to specialist)
Member --reportsTo--> Member (nurse to physician)
Member --diagnoses--> Member (physician to patient)
Obviously a Member may have as many of any edge type (e.g. multiple "consults" with specialists). In a more complex model, you might also see a Member as being an "identity" for a person such that your model looks like:
Member --actsAsPhysician--> Physician
Member --actsAsSpecialist--> Specialist
Physician --consults--> Specialist
In this approach the "consults" edge can only exist on a "Physician" vertex, thus you provide some constraints as to what vertex types can be expected to have what kind of edges.
Graphs provide you a lot of flexibility in being able to model data such as it exists in the real-world as you are really just describing entities and the relationships among them. I'd encourage you to look at http://tinkerpop.com for a collection of tools that are helpful in building graph applications independent of the graph database you choose.
If every one is a member then member is central to the data model in a relational perspective. There is no need for recursive SQL select statements:
Member ->---<- Doctor ->---<- Patients
One or More Members is One or More Doctors One or More Doctors treats One or More Patients
Your model will have a lot of Many to Many relationships and alot of Relationship tables. For instance the Treats relation could contain attributes such as Treatment Period
Ailment
etc.
Your solution could be implemented in any topology. While the network/graph topology is faster than the Set topology the graph data model once implemented is almost impossible to change. History tells us it is unwise to build rigid business applications. So research the pros and cons of each and make a decision.