or how can i represent an address to country mapping where an address has one one country in castle activerecord?
The [BelongsTo] attribute is there for one-to-one relations.
http://api.castleproject.org/html/T_Castle_ActiveRecord_BelongsToAttribute.htm
Related
My Entity Relationship Diagram
My ER diagram designed for the Orders database about customers and their orders for
a company. Each item
in the database must have an “owning” salesman, whether or not it is being ordered.
With that being said I was wondering whether there were any issues with my diagram? What do you suggest I should change?
Should I remove the customer and salesman attributes?
I not sure but I think in your the Address Entity is extra its a attribute customer only so why you create address as a separate attribute.
Please refer the attach image to correct your different relationship with attributes.
I am creating an ER diagram and relational schema for a school project. In my ER diagram, I have shown attributes like "street name" and "city" as weak attributes of another attribute "address." How would I convert this to a relational schema? Do weak attributes even appear in my relational schema?
Part of my ER Diagram
I prefer to call Billing Address a composite attribute, and City a component attribute.
The term "weak" is used in ER diagrams to describe entity sets that include another entity set's key as part of its own. For example, if an Invoice is identified by an Invoice_ID, and contains LineItems that are identified by Invoice_ID, Line_Number, that would make LineItem a weak entity set, and Line_Number would be its weak key.
When converting composite/component attributes to a relational schema, the simplest approach is to use only the component attributes. This means the composite attributes are nothing more than a visual aid.
My question is the following: Can relations have key attributes like shown in the following figure?
For me it doesn't make sense, however I have found them like in 1. If it is possilbe, how should I "resolve" them in the relational schema?
I found a similar the question on [2] but it seems to focus on how to handle attributes during the transformation of the ERM to the relational schema.
1 https://www.wu.ac.at/fileadmin/wu/processed/csm_erm_cardinalities2_84a65dbc2b.png
[2] relationship attributes in ER diagrams
According to Chen in The Entity-Relationship Model - Toward a Unified View of Data, a relationship set is an association among entity sets, while an attribute is a mapping from an entity set or relationship set to a value set. This means the entities that make up the relationship must be the determinant of the attribute, so a relationship can't depend on its own attributes.
The situation is complicated by common language use - people tend to use attribute to mean a column, which conflates attributes with value sets. Tables that represent relationship sets do have key columns, and those columns do represent attributes of the specific entity sets they represent, but they don't represent attributes of the relationship set.
Note that in your example [1], the key "attribute" on the relationship represents a composition of the keys of ABC and XYZ, so it isn't really a distinct attribute. Normally, in ER diagrams we understand that the keys of the associated entities determine the relationship, so there's no need to indicate a key directly on the relationship shape.
I want to add different type of departments to an entity called department in a ER diagram.Can I use ISA relationship for it.there are more than 2 departments.does ISA relationship can have more than 2 subclasses?
A supertype can have any number of subtypes. However, it's only useful to define subtypes if they'll have their own attributes or relationships. If not, a type attribute is sufficient.
I have checked out the cookbook web site of cakephp that there are four types Model relationship:
http://book.cakephp.org/view/79/Relationship-Types
Since the one I am more familiar with is belongsTo,
I am not sure when I need to use hasManay and HABTM.
What will be the result to my web site if I used a wrong Model relationship type?
Please advise.
belongsTo and hasMany are a pair and express a one-to-many relationship. One model belongs to another (i.e. it has a other_model_id field), while the other way around that other model has many records in the model that belongs to it. It's two sides of the same coin.
hasAndBelongsToMany is Cake's term for a many-to-many relationship. Here's a primer on this type of relationship. You use this when both models can have many of the other model, e.g. People-People friend relationships. A many-to-many relationship between two models entails three tables: model_a, model_b and model_a_model_b. If you're trying to use this type of relationship without the third table, you'll probably just get a bunch of errors.