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independent objectified fact type

Last post 10-24-2008 17:01 by rolemo. 6 replies.
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  • 10-23-2008 16:17

    independent objectified fact type

     Hi,

    I find it hard to understand why is it nessary to note the independacy of an objectified fact type.

     In the attached PNG for example, since Teaching doesnt have a mandatory dot we know its independant. so why is it important to point it out?

     Thanks


  • 10-23-2008 16:56 In reply to

    Re: independent objectified fact type

    Rolemo

    For uniformity, we decided to make the rules for objectified associations the same as for normal object types (by default, an object type is not independent). Suppose you had a schema consisting of just the fact type "Person(.id) was born in Country(.code)".  By default, Person and country are not independent, so their roles in this fact type are implied mandatory even if you don't declare them to be mandatory. Similarly, the role of Teaching in the example under discussion is implied mandatory unless you declare Teaching to be independent.

    This rule might seem strange at first, but you get used to it.

    Cheers

    Terry

  • 10-24-2008 3:02 In reply to

    Re: independent objectified fact type

    Hi Terry,

    Thanks for the explanation, but you are right, it does sound strange Cool

    It pops the obvious question - so why do we need the mandatory dot for? Put in other words - wouldn't it make sense then to  use an "IsIndependent Dot" instead of a mandatory dot, which is practically always there (by implication)?

    Cheers 

  • 10-24-2008 9:47 In reply to

    Re: independent objectified fact type

    Hi Rolemo

    In a typical business model, many roles are optional rather than being mandatory (e.g. It is typically mandatory for customers to have a name but optional for them to have a fax number). It is rare to have a simple object type independent, although fairly common to have an objectified association independent. The two constructs are both needed.

    Cheers

    Terry

  • 10-24-2008 14:32 In reply to

    Re: independent objectified fact type

    Hi Terry,

    Sorry, but I'm still confused. Would you kindly relate your explanation to this diagram?

     As i understand it, each Man can have at most one Hat, and each hat may belongs to more than one man. Are you saying that both roles are implied mandatory? So that each Man must have exactly one Hat, and each hat must belong to at least one Man?

    Thanks a lot! 


  • 10-24-2008 14:41 In reply to

    Re: independent objectified fact type

    Hi Rolemo

    If Hat and Man play no other roles in the global schema then Yes: because they have not been declared independent, the disjunction of their fact roles (in this case just one role for each) is mandatory.

    In a typical schema however, Man would play many more roles, so the single role of having a hat would not then be implied mandatory.

    Cheers

    terry

  • 10-24-2008 17:01 In reply to

    Re: independent objectified fact type

    Got it!

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