BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How to model relationships? Goto page 1, 2  Next          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Robert Cram Guest Posted: Wed Dec 01, 2004 9:15 am    Post subject: How to model relationships? Hi All, I'm trying to model (real life) relationships. In this case a person (class PERSON) can have 0..n relationships (class RELATIONSHIP) over time with 0..1 partners (class PERSON). I can't think of a way to model this in UML. Any thoughts on how to model the associations between the two classes PERSON and RELATIONSHIP in a class diagram? TIA, Robert. Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 9:58 am    Post subject: Re: How to model relationships? Joanna Carter (TeamB) wrote: Quote: "Robert Cram" <rcram (AT) knoware (DOT) nl> a icrit dans le message de news: 41ad99d1$1 (AT) newsgroups (DOT) borland.com... I'm trying to model (real life) relationships. In this case a person (class PERSON) can have 0..n relationships (class RELATIONSHIP) over time with 0..1 partners (class PERSON). I can't think of a way to model this in UML. Any thoughts on how to model the associations between the two classes PERSON and RELATIONSHIP in a class diagram? This is known as an 'association class' and is drawn as a loop from/to the person class with a dotted line from that loop to the relationship class. Joanna Hi Joanna, I think in this case it's not an association class because it's possible for example that a person is married to the same person twice, so the class RELATIONSHIP must have it's own identity. Back to top Joanna Carter (TeamB) Guest Posted: Wed Dec 01, 2004 10:35 am    Post subject: Re: How to model relationships? "Robert Cram" <rcram (AT) knoware (DOT) nl> a écrit dans le message de news: 41ad99d1$1 (AT) newsgroups (DOT) borland.com... Quote: I'm trying to model (real life) relationships. In this case a person (class PERSON) can have 0..n relationships (class RELATIONSHIP) over time with 0..1 partners (class PERSON). I can't think of a way to model this in UML. Any thoughts on how to model the associations between the two classes PERSON and RELATIONSHIP in a class diagram? This is known as an 'association class' and is drawn as a loop from/to the person class with a dotted line from that loop to the relationship class. Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 10:41 am    Post subject: Re: How to model relationships? Jim Cooper wrote: Quote: I think in this case it's not an association class because it's possible for example that a person is married to the same person twice, so the class RELATIONSHIP must have it's own identity. That's what Joanna is saying :-) Cheers, Jim Cooper Now I'm confused... I thought the essence of an association class is that it does NOT have its own identity. It gets its identity from the two objects that are associated (the assocation class models properties of the association). In this case the RELATIONSHIP class must have it's own identity (and therefore cannot be an association class) because the properties of the relationship are NOT the properties of an association between two persons (the same two persons can have more relationships over time). Back to top Jim Cooper Guest Posted: Wed Dec 01, 2004 11:07 am    Post subject: Re: How to model relationships? Quote: I think in this case it's not an association class because it's possible for example that a person is married to the same person twice, so the class RELATIONSHIP must have it's own identity. That's what Joanna is saying :-) Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 12:10 pm    Post subject: Re: How to model relationships? Joanna Carter (TeamB) wrote: Quote: "Robert Cram" <rcram (AT) knoware (DOT) nl> a icrit dans le message de news: [email]xn0dqhuy417kuab000 (AT) forums (DOT) borland.com[/email]... I thought the essence of an association class is that it does NOT have its own identity. It gets its identity from the two objects that are associated (the assocation class models properties of the association). In this case the RELATIONSHIP class must have it's own identity (and therefore cannot be an association class) because the properties of the relationship are NOT the properties of an association between two persons (the same two persons can have more relationships over time). On the contrary, an association class should have its own identity, otherwise it has no relevance to the association. In the scenario of a relationship between two different classes, you would typically have : Member -- Attendance -- Meeting Each Attendance records which member attended which meeting, thus resolving what is essentially a many to many relationship. This is exactly what you have with the possible relationships between many people. Joanna Hi Joanna, Perhaps it's a question of terminology. Let me try to explain my view: In your example attendance gets its identity from the combined identity of a certain member and a certain meeting. That's why it's an association class. There is always one attendance for one member/meeting combination. This is not true for relationships. There can be more relationships for one person/person combination (married twice to the same person), that's why it needs its own identity and can't be an associationclass. Am I making sense? Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 12:23 pm    Post subject: Re: How to model relationships? Bob Dawson wrote: Quote: "Robert Cram" wrote I thought the essence of an association class is that it does NOT have its own identity. From the RDBMS point of view, a row in a simple association table is uniquely identifiable from its composite key, and so needs no further identification. To allow for repetitious relationships, the DBA might fall back on a dumb key, but even then would look for some other tracked characteristice that provides uniqueness when added to the composite key--for example StartDate (you can marry and remarry the same person, but not at the same time). At that point the DBA is likely to stop regarding the table as a simple association any regard it instead as a true entity table. But from an OO point of view everything has an id, whether theoretically needed by the DBA or not. bobD Hi Bob, Ok, I understand that. I probably didn't use the right terminology by saying an asociationclass doesn't have an identity. I should have said: "an association class takes its identity from from the object references in it" (shamelessly copied from the UML reference manual) For relationships this is not the case. So in my view it can't be modelled as an association class. Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 12:29 pm    Post subject: Re: How to model relationships? Bjxrge Sfther wrote: Quote: Robert Cram wrote: I'm trying to model (real life) relationships. In this case a person (class PERSON) can have 0..n relationships (class RELATIONSHIP) over time with 0..1 partners (class PERSON). This design solution does have pitfalls...the "0..1" constraint being the most apparent case for a modification.... I wonder: is being single having a relationship with no one, having a relationship with oneself or not having a relationship at all? The real world is overly complicated ;) Back to top Joanna Carter (TeamB) Guest Posted: Wed Dec 01, 2004 12:58 pm    Post subject: Re: How to model relationships? "Robert Cram" <rcram (AT) knoware (DOT) nl> a écrit dans le message de news: [email]xn0dqhuy417kuab000 (AT) forums (DOT) borland.com[/email]... Quote: I thought the essence of an association class is that it does NOT have its own identity. It gets its identity from the two objects that are associated (the assocation class models properties of the association). In this case the RELATIONSHIP class must have it's own identity (and therefore cannot be an association class) because the properties of the relationship are NOT the properties of an association between two persons (the same two persons can have more relationships over time). On the contrary, an association class should have its own identity, otherwise it has no relevance to the association. In the scenario of a relationship between two different classes, you would typically have : Member -- Attendance -- Meeting Each Attendance records which member attended which meeting, thus resolving what is essentially a many to many relationship. This is exactly what you have with the possible relationships between many people. Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Bob Dawson Guest Posted: Wed Dec 01, 2004 1:09 pm    Post subject: Re: How to model relationships? "Robert Cram" wrote Quote: I thought the essence of an association class is that it does NOT have its own identity. From the RDBMS point of view, a row in a simple association table is uniquely identifiable from its composite key, and so needs no further identification. To allow for repetitious relationships, the DBA might fall back on a dumb key, but even then would look for some other tracked characteristice that provides uniqueness when added to the composite key--for example StartDate (you can marry and remarry the same person, but not at the same time). At that point the DBA is likely to stop regarding the table as a simple association any regard it instead as a true entity table. But from an OO point of view everything has an id, whether theoretically needed by the DBA or not. bobD Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 1:38 pm    Post subject: Re: How to model relationships? Jim Cooper wrote: Quote: In your example attendance gets its identity from the combined identity of a certain member and a certain meeting. Why? In Joanna's example it's true you wouldn't expect to have the same member attending a given meeting more than once, but that does not imply anything about identity. Typically objects have some other means of identification, eg memory address or a specific property (like OID) Cheers, Jim Cooper Hi Jim, I guess my use of the word "identity" has complicated things. The UML language reference manual probably states it more clearly: "An association class connecting classes A and B is not the same as a class D with binary associations to A and B. Like all links, a link of an association class such as C takes its identity from the object references in it. The attribute values are not involved in providing identity. Therefore, two links of C must not have the same pair (a,b) objects, even if their attribute values differ, because they would have the same identity. That is, given attribute E, it is not permitted that (a,b,e1) and (a,b,e2) both be instances of C, because they share the same identity (a,b)" "An association, including an association class such as C , is a set of tuples and has no duplicates among its object references; whereas, an implicit relationship such as D is more like a bag , which can have duplicates" That's why I concluded that the class RELATIONSHIP cannot be modelled as an association class. If I'm wrong, I'm at least in good company ;) Back to top Joanna Carter (TeamB) Guest Posted: Wed Dec 01, 2004 1:44 pm    Post subject: Re: How to model relationships? "Robert Cram" <rcram (AT) knoware (DOT) nl> a écrit dans le message de news: [email]xn0dqhxq21bb6l3003 (AT) forums (DOT) borland.com[/email]... Quote: Ok, I understand that. I probably didn't use the right terminology by saying an asociationclass doesn't have an identity. I should have said: "an association class takes its identity from from the object references in it" (shamelessly copied from the UML reference manual) For relationships this is not the case. So in my view it can't be modelled as an association class. There is no reason why an association class cannot have its own identity as well as holding the ids of its references. The UML docs are not intended to be rules set instone, they are guidance. Many good structures used in OO are not mentioned in the UML docs, see the Design Pätterns material available. If you insist on creating a 'key' out of the association then to make it unique, you would have to also include the start/end dates of the relationship; thus more fully describing the relationship. If an association class is nothing more than a pair (or more) of keys, then I would say that you are looking more at the RDBMS idea of resolving n-n relationships without adding the extra data that makes it a true association class. Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 2:05 pm    Post subject: Re: How to model relationships? Bob Dawson wrote: Quote: "Robert Cram" wrote "an association class takes its identity from from the object references in it" (shamelessly copied from the UML reference manual) Okay--from that point of view the relevant discussion is the "association class" entry in the 'Encyclopedia of Terms', UML Reference Manual (in my copy pp 157-9). See the last paragraph of the Discussion section and figure 13-28, 'Reified association.' Basically the three amigos model a reified association class as a normal class, so instead of putting it on a dotted line hanging off the primary association solid line, the association class appears directly on the solid line as an intermediary, reducing the multiplicity markings accordingly: E1|1_____*|AssociationClass|*_________1|E2| Or in your case Person |1_____*| Relationship | |1_____*| | Then there are a lot of complications that require either OCL or some other annotation, as --A relationship is non-reflexive (people generally don't marry themselves) --some types of relationships may be multiplicitous only over time (only one spouse at a time, depending on jurisdiction) --some types of relationships may be assymetric (spouse must be of opposite sex), again depending on jurisdiction --some types of relationships may be incompatible with relationships with the same person (a person might have both a spousal and an illicit lover relationship, but not with the same person) bobD Thanks Bob, I live in Holland, so thankfully some complications do not apply ;) One of my remaing problems is how to make explicit in this model that if A has a relationship to B, B has the same relationship with A. Back to top Robert Cram Guest Posted: Wed Dec 01, 2004 2:07 pm    Post subject: Re: How to model relationships? Bob Dawson wrote: Quote: "Robert Cram" wrote I wonder: is being single having a relationship with no one, having a relationship with oneself or not having a relationship at all? Best to remember at this point that the goal of the model is not to model the real world, but only to produce a model accurate enough to function for those aspects relevant to the problem. So the answer really is 'depends on what the program is trying to do.' The real world is overly complicated ;) Indeed. So the real question is not 'is this model right?', but rather 'will this model work for the task at hand?' Trying to model 'accurately' rather than 'appropriately' is a mistake. bobD Hi Bob, I know, this post was "in jest", as was (i hope) the post I replied to ;) Robert. Back to top Jim Cooper Guest Posted: Wed Dec 01, 2004 2:17 pm    Post subject: Re: How to model relationships? Quote: In your example attendance gets its identity from the combined identity of a certain member and a certain meeting. Why? In Joanna's example it's true you wouldn't expect to have the same member attending a given meeting more than once, but that does not imply anything about identity. Typically objects have some other means of identification, eg memory address or a specific property (like OID) Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> Delphi OO design All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.