Part 8 Link Objects association classes
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<h5> <strong> Deep overview of link objects and association classes </strong> </h5>
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  A Deep Overview of Link Objects and Association Classes
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    <span data-video="yDKHzI992jQ" data-start="1597" tabindex="0"> No ability to remove things from the  inner link </span>
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[[Category:MDriven Designer]]
Raw subtitles text
[[Category:MDriven Designer Sessions]]
 
[[Category:Associations]]
hi, we got a few questions on link objects or Association classes
{{Edited|July|12|2024}}
 
so, we're going to dive into those right now
 
this is a typical association class
 
it is on a many-to-many association
 
let's draw this in a new diagram
 
going just move these things
 
"person", "house, "lease contract"
 
not move them, we're going to view them in a separate diagram
 
create a new diagram
 
I open that diagram
 
I find what I was looking for house class, person class
 
as I drag this on, all the associations
 
associated with these two classes are also drawn
 
and since the association class is part of this
 
association it was implicitly added to my view
 
this is, of course, just what
 
I want now, so that's ok
 
if I didn't want it, I would mark it
 
and press "Delete"
 
remember, if we press Delete we just
 
remove it from the diagram
 
"Ctrl+Delete" delete from the model completely
 
this association, I'm not interested to show in this diagram
 
so I press delete and not "Ctrl+Delete"
 
because that will remove it and I don't
 
want that, that would remove it from the model
 
I just want to remove it from the diagram,
 
delete, all right
 
let's look at this, what happens
 
the house has many tenants that are of class person
 
the person has many rented apartments
 
that are of class house
 
in every relationship that is many-to-many, like this
 
we have the ability to add
 
association class, like this
 
it's added with the dotted line tool, this one
 
let's check the tooltip drag-out
 
new association class connection
 
so if I press that one, I can find association
 
but I had one already, so I can remove the
 
association class by clicking the association line
 
find in the property inspector the class
 
that is selected
 
and I can choose to have no association class
 
when I do this the dotted line is removed
 
I can set it back by choosing it,
 
but the dotted line would not get there
 
because he and it's only
 
drawn, when added to the diagram
 
if it is a association, so a better way,
 
now to get it to show
 
I can remove this from view
 
and say right-click, introduce&removed...
 
that ended up outside the screen
 
still, introduce&removed, pick from available
 
when I remove this might
 
as well, just to make it easier for me
 
introduce pick from available own houses
 
rented apartments
 
you can see it, but I see it
 
I don't know how to get this on screen
 
so now it added the thing like this
 
so now we're back, where we started from
 
that was around about way
 
sorry about that
 
so these are the association ends
 
rented apartments and tenants
 
and these is the association
 
so on the association I see the ends
 
rented apartments and tenants
 
if I click one, I only see this association end
 
and the other one and I can see that
 
name property to tenants
 
if I click the other one, we see
 
the name property rented apartments
 
what about this, what actually happens is
 
there is an implicit link available in OCL for us
 
so that we can go from the house
 
to the lease contracts,
 
because the lease contracts are often quite good to use
 
and it has important information in this case
 
apartment number, end date and rent, etc.
 
so let's see how a viewmodel that
 
started in house and say that it had all houses
 
so I would add a nested...with
 
multilinks, house allinstances
 
ok, so this will be a list where I have all the houses
 
and for certain house I can say that
 
I want to show the street number
 
and for the selected house
 
I can see that I have a few multi-link options
 
is that onscreen for you
 
the tenants, which is this one
 
but also the lease contract which is that one
 
alright, so
 
if I add a list of those lease contract
 
self lease contract it ended up a bit missplaced
 
but let's say that for each house
 
each selected the house
 
I have a list of lease contracts
 
so let's add a few columns to that grid
 
that apartment number and rent
 
so what we've learned is that there is an implicit
 
link to get here
 
why is that link called lease contract
 
good question
 
actually, when we stand on the association
 
we fold down the one called tenants
 
we have something we called inner link name
 
and link role name
 
these are both to default and
 
default means that it's gonna
 
and get a name based on the class
 
it's pointing to basically
 
and remember that the one used
 
was called lease contract
 
why was it called lease contract
 
well it turns out it's the link role name
 
so if I were to change this to
 
the lease contract the ocl editor
 
would say that you use the name lease contract
 
I don't even know what that is
 
but the lease contracts that I know
 
so let's remove that one and type the lease contracts
 
and that's a collection of lease contract
 
so this is actually a better naming
 
of lease contract list
 
so instead of the default, but that was
 
when I point at this end "tenants"
 
I can really go this way, with the link role name
 
the lease contract
 
but once here, I might want to continue
 
to go up this way
 
and go that way
 
let's see, so I'm in a lease contract
 
going to add a generic empty column
 
used to get an OCL position to writing
 
and see roles, there is one called tenants person
 
and that one, the tenants going to
 
use that one, that's just...
 
that points out one person, but maybe
 
we think that is a good name
 
it's actually quite ok name, but just
 
for the sake of arguments
 
we're gonna change the inner link name
 
this is called the inner link
 
to the interlink name, let's call it
 
the tenant of the lease
 
and if I have done that and check this OCL again
 
it says, oh I don't know anything about
 
tenants from lease contract
 
but I do know the tenant of the lease,
 
so that's still the new name that we gave it
 
so what we have learned is of course
 
we can go straight to the tenants
 
it will be a list of persons,
 
but we can also go to
 
the link role name, to the lease contract
 
in this case, we called it the lease contracts
 
then we are here and then we might want
 
to go up here and to the specific person
 
associated with this lease contract and
 
that would be called the tenant of the lease
 
ok so important things with link objects
 
or association classes or that they are
 
life time controlled by the framework
 
this is really part of the uml standard
 
that whenever a person is associated
 
with the house, in this association
 
namely the person gets another rented apartment
 
if we add a house to the person's rented apartments
 
a lease contract will implicitly be created
 
we can't avoid it, that is what the link class does
 
whenever there's a new of object
 
added in this end of the association
 
and new link oobject is created in of this class
 
and the same is if we had a this class
 
and were to add it in person as a tenant
 
it would create at lease contract
 
so that means that we can always trust
 
that there will be at lease contract
 
where we have designed it this way
 
ok, but if that is the case that it is
 
lifetime controlled
 
by the existence of this association
 
and how can we then delete or remove a lease contract
 
instead we should remove the person
 
as a tenant, if we remove a person
 
as a talent, the lease contract
 
associated with that person will be removed
 
and vice versa, if a person would
 
remove a rented apartment
 
the lease contract would be destructed
 
logically the association class could be simulated this way
 
if we have a multilink of
 
lease contracts that points out one house
 
and we also had a multi-ink from
 
person to lease contracts that as one person
 
this is more or less equivalent
 
to this association class, but this
 
implementation of the same concept does not offer
 
the lifetime control that this
 
implementation does
 
this is often how we see association class
 
is done in databases
 
but the databases seldom offer their lifetime control
 
actually, when we do it
 
with an association class we cover a lot of ground
 
with very few concepts
 
this tells the reader of the model
 
that the lease contracts are
 
implicitly created whenever there's a house
 
and person in association
 
lets use set the specific attributes on that
 
it's a real strong concept of modeling and abstraction
 
so we're gonna try this out
 
I'm going to remove this column and
 
add an action to show this
 
just a dummy action add, global action for show
 
then I got that this action will be created
 
ok, save this and see why its
 
yeah, since I changed the innerlink names
 
I will need to change the places
 
where I use them
 
so, this could be a good example of
 
so, we are on a lease contract, but we want
 
the tenant of the lease, self.. instead... and
 
we had more errors, so instead of calling it
 
that it was called the lease contract
 
so I corrected those, start prototyping
 
and bring up our sample prototype and
 
see there's an exception null value in
 
link object, because we loaded the file
 
where there were missing data
 
since we changed it in the XML file
 
we don't evolve the data,
 
so if we change names
 
we can get and strange results like this
 
but the xml file is mainly suitable
 
for prototyping data so that's not a problem for us
 
we can just clean that up,
 
so I cleaned up the file, out of the screen
 
I basically recreated a file
 
so this was the one we created
 
we can see that, we see just this street
 
number and the apartment number
 
that might be a little vague
 
can we add something more, so that we get
 
some context, the street
 
model refresh, view, the street name,
 
if I pointed this one
 
I can see that I have a few lease contracts
 
but on each lease contract
 
I can add a generic column following to
 
the tenant of the lease and full name
 
like that, want to call that "full name", like that
 
so, if I kwould want to remove one of
 
these lease contracts
 
how would I go about that
 
well, let's say that we want to add a button
 
to remove lease contract
 
this time I'm gonna add a generic volume here
 
and I'm gonna check "is action"
 
it ended up there, I'm going to add here
 
don't need to be that big, like that
 
"Delete Picked Lease", so which one is the
 
currently picked lease
 
I know they are in this list, lease contract list
 
so there should be a variable called "vCurrent
 
lease contract list"
 
and let's try with a delete
 
remember the thing I said that shouldn't work
 
because this is actually lifetime
 
controlled by the framework
 
model-refresh, select, select this one and delete
 
and that's you saw that, went just fine
 
so actually I remembered wrong
 
it's ok to delete the link objects
 
because what we will do is to
 
clean up these two associations
 
you can do delete this, what I was referring to
 
is that we cannot remove things from the inner link
 
so let's try that next
 
because this has no possibility
 
to live without the house and the person
 
so the house and the person is the reason
 
for this existence, for its existence
 
so this was ok, but I'm gonna undo
 
we had that one right, undo, so "Ctrl+Z"
 
cancel on everything
 
Discard changes? - yes,
 
so let's head back to the view model
 
if I right click here
 
and insert row, I get a new in the grid
 
everything is moved down one step for me
 
so that's a convenient way
 
to get more space
 
so I don't have to move everything by hand
 
but gonna copy this action "ctrl-c", "ctrl-v" and
 
remove from inner link
 
this is what shouldn't be possible
 
so from instead the selected house
 
vCurrent all house
 
this is the selected house, vCurrent is one
 
the current one and don't confuse that
 
this with vSelected all house
 
that contains all houses, that are selected
 
so basically when I do this
 
hold ctrl down and select another one
 
can't do that because we are in edit mode here
 
when I do this I have houses multiple houses selected
 
and they are all in the vSelected all house collection of houses
 
but we just want to the current house
 
the currently selected, the currently focused house,
 
that's in the vCurrent all house
 
like that and the inner link
 
that we gave a name, explicit name
 
the lease contracts
 
we can try to remove
 
here we could use the currently focused lease contract
 
vCurrent lease contact list
 
that's the one that is the current in this list
 
so this would be illegal
 
if what I say is correct
 
now they are on the same spot,
 
I'm going to move this one here
 
remove from inner link
 
head back over here, model refresh,
 
show the view model again
 
now this one is selected and now that one is also
 
this one is vCurrent for all house
 
this is the current for lease contract list
 
remove from inner link error and
 
list of link objects is read-only
 
list of links object is read-only
 
that was where I was going for to show you
 
so that it's not allowed, because we can't
 
remove things in this implicit link
 
instead we would need to remove this one
 
from this link
 
so if we were to rectify this
 
let's do yet another copy
 
remove from correct link, we could use
 
the same vCurrent all house
 
but then it should be the role to tenants
 
from here we want to remove the vCurrent lease contract
 
but that will not be correct because that's
 
of the wrong type it expects a tenant
 
so we need to follow the inner link up to
 
the tenant of the lease
 
now we have the tenant of the lease
 
in the tenants collection
 
so the buttons ended up on top of each other
 
when I copy, going to move it here
 
so now I'm removing one of
 
these from the selected of those
 
selected of those following the inner link to here
 
inner link name, the tenant of the lease that name
 
and the link role name, that name
 
so there's an implicit name here and one here
 
I showed these in the second grid
 
but in order to remove, to delete this one
 
by removing
 
I need to remove one of these from this link
 
not really that confusing once you get the hang of it
 
model refresh, so I picked that one
 
and I picked this one
 
now I'm going to remove from correct
 
link and then it disappears and
 
what happens is that since one of these
 
objects are removed from this link
 
this one has no, one of these loses its right to exist
 
so it is destroyed by the framework
 
get some warnings, because they
 
should have apartment numbers
 
yeah, that's okay, the ones missing
 
apartment numbers were these step
 
or any other ones, these have,
 
ok so quite an in-depth check on Association classes
 
and how they work
 
let me know if you
 
have further questions
 
and we will dive into those as well thank you

Latest revision as of 15:44, 10 February 2024

A Deep Overview of Link Objects and Association Classes

To make your experience smooth, we set the main tags mentioned in the video to the right bar menu of this mini-player. Choose an interesting subtitle on the list and immediately get to the exact theme navigation item place in the video. Now you can pick any topic to be instructed on without watching the whole video.

Typical association class Association class with the dotted line Introduce and remove Association ends Implicit link Inner link name and link role name Lifenavigation-item controlled objects and classes Concept of modelling and abstraction No ability to remove things from the inner link Remove from the correct link

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