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
Related
Situation: In the app we have up to 1000 schools. Every school has students and students are having lessons and are joining events (and more). We need to query quick and often lessons per student, per school per date. We have 2 designs in mind, wondering the best way to proceed.
1 - design with dedicated school node
2 - design with no dedicated school node
Examples of two designs
PRO design 1
- root ref to school user after login. noo need to query on school id's
- no need to mention school id's everywhere
- no need for node lessens per school and events per school
- rules on school level
...
PRO design 2
- more flatten data, as widely advised on the internet
For most NoSQL database structures, flattening and denormalising data is the best method. And that is exactly the case with Firebase too.
When you flatten your data, you get the following advantages :-
You're mostly only downloading the minimum required amount. That leads to efficiency and cost-effectiveness.
Your downloads are much faster - specially compared to the likes of SQL join queries.
Having said that, in your particular case, I think that it really depends on how much the school affects the logged in user.
Suppose that a school is only an attribute for a student, and serves no other purpose, then the second database is the way to go. For example, if the books a student can get are independent of the school she goes to, then the second database style is more suited.
However, if a school categories students into groups that define their interaction with the database, then the first database structure is the way to go. An example of this is that a student can only get a book when its available in the school she goes to.
Regardless of your decision, I would like to commend you on the fact that you have flattened your database quite well in both your structures! And my personal suggestion would be to go with the one that is more convenient to code, read and maintain for you.
There is one example to explaining associations in UML.
A person works for a company; a company has a number offices.
But I am unable to understand the relationship between Person, Company, and Office classes. My understanding is:
a company consists of many persons as employees but these classes exist independently so that is simple association with 0..* multiplicity on Person class' end
a company has many offices and those offices will not exist if there is no company so that is composition having Company as the parent class and 0..* multiplicity on Branch class' end.
But I am not sure of 2nd point. Please correct me if I am wrong.
Thank you.
Why use composition or aggregation in this situation at all? The UML spec leaves the meaning of aggregation to the modeler. What do you want it to mean to your audience? And the meaning of composition is probably too strong for this situation. Thus, why use it here? I recommend you use a simple association.
If I were you, I would stay truer to the problem domain. In the world I know, Offices don't cease to exist when a Company goes out of business. Rather, a Company occupies some number of Offices for some limited period of time. If a Company goes out of business, the Offices get sold or leased to some other Company. The Offices are not burned to the ground.
If you aren't true to the problem domain in an application, then the shortcuts you take will become invalid when the customer "changes the requirements" for that application. The problem domain doesn't actually change much, just the shortcuts you are allowed to take. If you take shortcuts to satisfy requirements in a way that are misaligned with the problem domain, it is expensive to adjust the application. Your customer becomes unhappy and you wind up working overtime. Save yourself and everyone the trouble!
While Jim's answer is correct, I want to add some extra information. There are two main uses for aggregation
Memory management
Database management
In the first case it gives a hint how long objects shall live. This is directly related to memory usage. If the target language is one which (like most modern languages) uses a garbage collector, you can simply ignore this model information.
In the second case, it's only partially a memory question. A composite aggregation in a database indicates that the aggregated elements need to be deleted along with the aggregating element. This is less a memory but in most cases a security issue. So here you have to think twice.
A shared aggregation however has a very esoteric meaning in all cases.
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.
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.
In my OLTP database I have a layout consisting of instructors and students. Each student can be a student of any number of instructors. A student can also sign up for an instructor, but not necessarily book any tuition (lesson).
In a data warehouse, how best would this be modelled? If I create a dimension table for Lessons, Instructors and Students and a fact table for the lessons students have taken then this will work when an instructor wants to see what lessons a student has taken.
However, how will an instructor see how many students are REGISTERED with the instructor but has not yet taken a lesson?
In my OLTP, I have a many to many table (InstructorStudents) that links each student with one more more instructors. In an OLAP database, this isn't appropriate.
What would be the best schema in this case? Would a many to many be appropriate in this instance? I can't store a list of which students are registered to which instructors in the student table, so I feel another dimension table is necessary but cannot work out what should be contained in it.
If a fact represents a transaction, you seem to have two different facts here: Sign ups & Lessons. There are always a lot of ways to go but, perhaps, you need two fact tables. They may have similar dimensionality except the sign-up table will have a Class dimension (class name, instructor name, etc.). The Lessons table will tie to the class dimension but, also, to a Lesson dimension (date, classroom used, etc.).
There are a few other ways to do this but they will be more difficult from a programming & reporting perspective.
You need a many to many dimensional model.
You need a factless fact table. Look at the following resource that refers to an example close to your need
http://www.kimballgroup.com/1996/09/02/factless-fact-tables/