Re: ManyToManyField is this right?

[David Wagner]

Thanks Tom. Is this the example your talking about,
https://docs.djangoproject.com/en/1.4/topics/db/models/#intermediary-manytomany?
If so, I think this should work...

# Different groups and states that issue teaching certificates/licenses
class IssuingAuthority(models.Model):
name = models.CharField(max_length=254)
state = USStateField()
date_added = models.DateTimeField(auto_now_add=True)

# Certifications issued related to IssuingAuthority
class Certifications(models.Model):
certs = models.ManyToManyField(License_Types, through='UserProfile')
name = models.CharField(max_length=254)
date_added = models.DateTimeField(auto_now_add=True)

# User Profile Model
class UserProfile(models.Model):
user = models.OneToOneField(User)
address_1 = models.CharField(max_length=254)
address_2 = models.CharField(max_length=254)
state = USStateField()
zip = models.IntegerField(max_length=7)
phone = PhoneNumberField()
issuing_authority = models.ForeignKey(IssuingAuthority)
certification = models.ForeignKey(Certifications)

I'll try to load that up later (most likely tomorrow) and see if I can get
it to work.

Thanks much.

On Wed, Jun 27, 2012 at 5:27 PM, Thomas Lockhart wrote:

>  On 6/27/12 11:48 AM, David Wagner wrote:
>
> Looking at that I think I may need to add a foreignkey to cert_types
> relating to person since a person can have multiple certification types
> (NRA Instructor, CCL Instructor, etc).
>
> No. See below.
>
>
> On Wed, Jun 27, 2012 at 11:40 AM, David Wagner wrote:
>
>> i think I may just be over thinking this. The last time I did any
>> significant coding for the web as pre-php5 and so this whole MVC thing is
>> something to adapt too for sure.
>>
>  Yes you are :)
>
>
>> I think I need to start thinking of the Model in the same way I would
>> design a database back in the day with phpMyAdmin. I think I'm getting
>> bogged down in trying to understand how it will relate to the View. Perhaps
>> I need to just put the View out of my mind for the time being.
>>
>  Yes. If you've done databases (that is some of my background too) then
> the models (always the first step here) should become comfortable fairly
> quickly, even if you are rusty. See below...
>
>
>> So thinking of this as just a database schema it would be something like
>> (in psuedo-code)
>>
>> cert_types
>>
>>- type
>>- date_created
>>
>> certs
>>
>>- type = foreignkey(cert_types)
>>- name
>>- state (optional)
>>- date_created
>>
>> person
>>
>>- name
>>- etc...
>>- certificates = foreignkey(certs)
>>
>>   OK, "cert_types" is good (or at least a good start). "person" needs to
> have a manytomany on cert_types rather than a foreign key on certs. And
> "certs" can be the explicit intermediate table used for the manytomany
> relationship and can hold things like dates. Look at the django docs on how
> to explicitly define the intermediate table, but it is in the musician and
> musical group example; google for "paul ringo django extra fields" :)
>
> hth
>
>  - Tom
>
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Re: ManyToManyField is this right?

[Thomas Lockhart]

On 6/27/12 11:48 AM, David Wagner wrote:
Looking at that I think I may need to add a foreignkey to cert_types 
relating to person since a person can have multiple certification 
types (NRA Instructor, CCL Instructor, etc).

No. See below.


On Wed, Jun 27, 2012 at 11:40 AM, David Wagner > wrote:


i think I may just be over thinking this. The last time I did any
significant coding for the web as pre-php5 and so this whole MVC
thing is something to adapt too for sure.


Yes you are :)



I think I need to start thinking of the Model in the same way I
would design a database back in the day with phpMyAdmin. I think
I'm getting bogged down in trying to understand how it will relate
to the View. Perhaps I need to just put the View out of my mind
for the time being.

Yes. If you've done databases (that is some of my background too) then 
the models (always the first step here) should become comfortable fairly 
quickly, even if you are rusty. See below...



So thinking of this as just a database schema it would be
something like (in psuedo-code)

cert_types

  * type
  * date_created

certs

  * type = foreignkey(cert_types)
  * name
  * state (optional)
  * date_created

person

  * name
  * etc...
  * certificates = foreignkey(certs)

OK, "cert_types" is good (or at least a good start). "person" needs to 
have a manytomany on cert_types rather than a foreign key on certs. And 
"certs" can be the explicit intermediate table used for the manytomany 
relationship and can hold things like dates. Look at the django docs on 
how to explicitly define the intermediate table, but it is in the 
musician and musical group example; google for "paul ringo django extra 
fields" :)


hth

 - Tom

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Re: ManyToManyField is this right?

[David Wagner]

Ah, I think that makes sense. Sorry if I wasn't clear earlier on but yes, I
am looking for instructors who are certified to teach a certain course. So
basically a user could sign up and select some or none of the types of
courses they are certified to teach this way other users who want to learn
can find them easily. As far as the difference between NRA and CCL, there
are several different types of NRA courses one could take however a
Concealed Carry course is dependent on which state your certified to teach
in. This is what was getting me all confused as I was trying to abstract
the type of course from the actual courses provided since there are a
number of organizations and individuals (not attached to an organization)
that can provide not only courses but services, such as a state certified
hunting guide (for instance in Alaska you need one to hunt large game).

I guess though from your example data structure I could put any type of
organization, state, etc into the IssuingAuthority and when it comes time
to create the profile forms I could just create a drop down list of all
issuing authorities for a person to choose from and then populate a select
box with all classes that belong to that issuing authority. I would like
for people to be able to multi-select and have fields auto-populate all on
one page at one time but I'll jump that hurdle when I get there I suppose.
Probably need to dive into Javascript/Ajax to make that work.

Thanks much for your help.



On Wed, Jun 27, 2012 at 3:31 PM, Dennis Lee Bieber wrote:

> On Wed, 27 Jun 2012 11:48:22 -0700, David Wagner 
> declaimed the following in gmane.comp.python.django.user:
>
> > Looking at that I think I may need to add a foreignkey to cert_types
> > relating to person since a person can have multiple certification types
> > (NRA Instructor, CCL Instructor, etc).
>
> Well, this has just added a different aspect... That of INSTRUCTOR
> (my memory of the first messages came across more as a certificate that
> one had attended a course , not that it meant they could teach the
> course).
>
>In the case of instructors -- what is the difference between "NRA"
> and "CCL" instructor? "NRA" specifies an organization that provides the
> certification in /something/, but "CCL" is a /something/ but who issues
> it?
>
>Okay, looking at these as teaching certificates...
>
> Certificates
>ID #primary key
>Name#text string for the common name of the class
>IssuingAuthority#NRA or government entity
>DateFirstAuthorized #not date of issuance, but a double check
> that
>#an issued
> certificate date does not come
>#before the "class"
> was created
>RenewalPeriod   #how often the certificate needs to be
> renewed
>
> Persons
>ID
>Name
>Address
>whatever
>
> Issued  #or PersonsCertificates if you want to name the
> sources
>ID
>pID foreign key (Persons.ID) #who this certificate
> was issued to
>cID foreign key (Certificates.ID)   #what type of
> certificate
>DateIssued  #constraint DateIssued > DateFirstAuthorized
>CertificateNumber   #document number
>DateExpires #for things that need to be renewed
> periodically
>#= DateIssued +
> RenewalPeriod
>UniqueKey (pID, cID, CertificateNumber)
>#including certificate number means you can track renewals
>#as a renewal would have a different serial number
>
>This is a many-to-many between Persons and Certificates, in which
> the intersection table (Issued) contains additional information.
>
>
> --
>Wulfraed Dennis Lee Bieber AF6VN
>wlfr...@ix.netcom.comHTTP://wlfraed.home.netcom.com/
>
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Re: ManyToManyField is this right?

[David Wagner]

Looking at that I think I may need to add a foreignkey to cert_types
relating to person since a person can have multiple certification types
(NRA Instructor, CCL Instructor, etc).

On Wed, Jun 27, 2012 at 11:40 AM, David Wagner  wrote:

> i think I may just be over thinking this. The last time I did any
> significant coding for the web as pre-php5 and so this whole MVC thing is
> something to adapt too for sure.
>
> I think I need to start thinking of the Model in the same way I would
> design a database back in the day with phpMyAdmin. I think I'm getting
> bogged down in trying to understand how it will relate to the View. Perhaps
> I need to just put the View out of my mind for the time being.
>
> So thinking of this as just a database schema it would be something like
> (in psuedo-code)
>
> cert_types
>
>- type
>- date_created
>
> certs
>
>- type = foreignkey(cert_types)
>- name
>- state (optional)
>- date_created
>
> person
>
>- name
>- etc...
>- certificates = foreignkey(certs)
>
> Does that make sense? If so each table would be represented by a single
> class and I think I understand then how they relate to each other and I
> just need to stop worrying about the View.
>
> Thanks for all your patience everyone as well. Like I said, I'm not
> classically trained in computer science so there are a lot of concepts that
> elude me and some lingo that I don't get but the concepts I may grasp if I
> just understand the translation.
> On Wed, Jun 27, 2012 at 10:55 AM, Dennis Lee Bieber  > wrote:
>
>> On Wed, 27 Jun 2012 08:02:37 -0700, David Wagner 
>> declaimed the following in gmane.comp.python.django.user:
>>
>> >
>> > class NRA_Certs(models.Model):
>> > CRSO = models.BooleanField(blank=True, null=True, "Chief Range
>> Safety
>> > Officer")
>> > HFS = models.BooleanField(blank=True, null=True, "Home Firearm
>> Safety")
>> > MCR = models.BooleanField(blank=True, null=True, "Metallic Cartridge
>> > Reloading")
>> > PPH = models.BooleanField(blank=True, null=True, "Personal
>> Protection
>> > in the Home")
>> > PPO = models.BooleanField(blank=True, null=True, "Personal
>> Protection
>> > Outside the Home")
>> > PS = models.BooleanField(blank=True, null=True, "Pistol Shooting")
>> > RS = models.BooleanField(blank=True, null=True, "Rifle Shooting")
>> > SSR = models.BooleanField(blank=True, null=True, "Shotgun Shell
>> > Reloading")
>> > SS = models.BooleanField(blank=True, null=True, "Shotgun Shooting")
>> > RTBVDW = models.BooleanField(blank=True, null=True, "Refuse to be a
>> > Victim Workshop")
>> > RTBVO = models.BooleanField(blank=True, null=True, "Refuse to be a
>> > Victim Online")
>> > NMLRA_MP = models.BooleanField(blank=True, null=True, "NMLRA
>> > Muzzleloading Pistol Shooting")
>> > NMLRA_MR = models.BooleanField(blank=True, null=True, "NMLRA
>> > Muzzleloading Rifle Shooting")
>> > NMLRA_MS = models.BooleanField(blank=True, null=True, "NMLRA
>> > Muzzleloading Shotgun Shooting")
>> >
>> Ugh!...
>>
>>I'm not familiar enough with Django's internal format so I'm using
>> a
>> form of old relational theory notation: tablename(_key_, /foreignkey/,
>> other, fields...)
>>
>> NRACertificate(_ID_, certificateName, other, type, specific,
>> attributes))
>>
>>{I forget your individual model so a generic}
>>
>> Person(_ID_, name, other, stuff)
>>
>> Person_NRACertificate(_ID_, /personID/, /nracertificateID/,
>> dateCertified, dateExpires, other, certificate, specfic, data)
>>
>>Django can build this intersection table automatically -- but it
>> would only have _ID_, /personID/, /nracertificateID/! I think, for your
>> information, you'd want to define your own "through" table to hold the
>> data specific to an instance of a certificate as held by a person.
>>
>>Same with concealed carry permits IF you need a list on a, say, per
>> state basis.
>>
>>Note though, that if the /lists/ don't carry any significant
>> information beyond the name of the item, you just need one-to-many
>> configurations. For example, if the only information you track for CCW
>> is: who (foreign key to Person), license number, issuing state (or other
>> government entity), date issued, date expires... then a one-to-many is
>> all you need. (Though you may find you need another one-to-many --
>> California's rare CCW includes a list of /which/ firearm(s) are
>> permitted [if you sell one gun and buy a different one, you have to beg
>> the government to update the CCW with the new gun's information]).
>>
>>The NRACertificates table may also not be needed if all it provides
>> is a list of names to populate a dropdown selection box -- might speed
>> up processing by just hard-coding the list and saving the name directly
>> (replace the /nracerticateID/ in the intersect table with the actual
>> text of the certificate as selected from the dropdown selection.
>>
>>Bigger note: You

Re: ManyToManyField is this right?

[David Wagner]

i think I may just be over thinking this. The last time I did any
significant coding for the web as pre-php5 and so this whole MVC thing is
something to adapt too for sure.

I think I need to start thinking of the Model in the same way I would
design a database back in the day with phpMyAdmin. I think I'm getting
bogged down in trying to understand how it will relate to the View. Perhaps
I need to just put the View out of my mind for the time being.

So thinking of this as just a database schema it would be something like
(in psuedo-code)

cert_types

   - type
   - date_created

certs

   - type = foreignkey(cert_types)
   - name
   - state (optional)
   - date_created

person

   - name
   - etc...
   - certificates = foreignkey(certs)

Does that make sense? If so each table would be represented by a single
class and I think I understand then how they relate to each other and I
just need to stop worrying about the View.

Thanks for all your patience everyone as well. Like I said, I'm not
classically trained in computer science so there are a lot of concepts that
elude me and some lingo that I don't get but the concepts I may grasp if I
just understand the translation.
On Wed, Jun 27, 2012 at 10:55 AM, Dennis Lee Bieber
wrote:

> On Wed, 27 Jun 2012 08:02:37 -0700, David Wagner 
> declaimed the following in gmane.comp.python.django.user:
>
> >
> > class NRA_Certs(models.Model):
> > CRSO = models.BooleanField(blank=True, null=True, "Chief Range Safety
> > Officer")
> > HFS = models.BooleanField(blank=True, null=True, "Home Firearm
> Safety")
> > MCR = models.BooleanField(blank=True, null=True, "Metallic Cartridge
> > Reloading")
> > PPH = models.BooleanField(blank=True, null=True, "Personal Protection
> > in the Home")
> > PPO = models.BooleanField(blank=True, null=True, "Personal Protection
> > Outside the Home")
> > PS = models.BooleanField(blank=True, null=True, "Pistol Shooting")
> > RS = models.BooleanField(blank=True, null=True, "Rifle Shooting")
> > SSR = models.BooleanField(blank=True, null=True, "Shotgun Shell
> > Reloading")
> > SS = models.BooleanField(blank=True, null=True, "Shotgun Shooting")
> > RTBVDW = models.BooleanField(blank=True, null=True, "Refuse to be a
> > Victim Workshop")
> > RTBVO = models.BooleanField(blank=True, null=True, "Refuse to be a
> > Victim Online")
> > NMLRA_MP = models.BooleanField(blank=True, null=True, "NMLRA
> > Muzzleloading Pistol Shooting")
> > NMLRA_MR = models.BooleanField(blank=True, null=True, "NMLRA
> > Muzzleloading Rifle Shooting")
> > NMLRA_MS = models.BooleanField(blank=True, null=True, "NMLRA
> > Muzzleloading Shotgun Shooting")
> >
> Ugh!...
>
>I'm not familiar enough with Django's internal format so I'm using a
> form of old relational theory notation: tablename(_key_, /foreignkey/,
> other, fields...)
>
> NRACertificate(_ID_, certificateName, other, type, specific,
> attributes))
>
>{I forget your individual model so a generic}
>
> Person(_ID_, name, other, stuff)
>
> Person_NRACertificate(_ID_, /personID/, /nracertificateID/,
> dateCertified, dateExpires, other, certificate, specfic, data)
>
>Django can build this intersection table automatically -- but it
> would only have _ID_, /personID/, /nracertificateID/! I think, for your
> information, you'd want to define your own "through" table to hold the
> data specific to an instance of a certificate as held by a person.
>
>Same with concealed carry permits IF you need a list on a, say, per
> state basis.
>
>Note though, that if the /lists/ don't carry any significant
> information beyond the name of the item, you just need one-to-many
> configurations. For example, if the only information you track for CCW
> is: who (foreign key to Person), license number, issuing state (or other
> government entity), date issued, date expires... then a one-to-many is
> all you need. (Though you may find you need another one-to-many --
> California's rare CCW includes a list of /which/ firearm(s) are
> permitted [if you sell one gun and buy a different one, you have to beg
> the government to update the CCW with the new gun's information]).
>
>The NRACertificates table may also not be needed if all it provides
> is a list of names to populate a dropdown selection box -- might speed
> up processing by just hard-coding the list and saving the name directly
> (replace the /nracerticateID/ in the intersect table with the actual
> text of the certificate as selected from the dropdown selection.
>
>Bigger note: You will NOT be using a multiple selection list... More
> reasonable is an "add new certificate" button which brings up a form
> from which to select one certificate (by name in dropdown), and fields
> for the dates, and whatever else is common to certificates.
>
>When displaying a person, the certificates would be (if I recall
> Django terms) a "formset" (where the "form" is for a sin

Re: ManyToManyField is this right?

[Thomas Lockhart]

On 12-06-27 8:02 AM, David Wagner wrote:
Thanks for all the replies and I think I'm getting it now but it gets 
more complex too.


So NRA Licenses is essentialy going to become a type of several 
certifications. For instance.


NRA Instructor

  * Home Firearm Safety Instructor
  * Refuse to be a Victim Workshop
  * and many more could be selected.

CCL (or CCW in some states)

  * State in Which your Certified (possibility of multiple states)

Many more different types of licenses and certifications are going to 
be added eventually so as you can see these relationships are going to 
get complex. So If I'm grasping this correctly let me try some code to 
see if I'm on the right track


class NRA_Certs(models.Model):
CRSO = models.BooleanField(blank=True, null=True, "Chief Range 
Safety Officer")
HFS = models.BooleanField(blank=True, null=True, "Home Firearm 
Safety")
MCR = models.BooleanField(blank=True, null=True, "Metallic 
Cartridge Reloading")
PPH = models.BooleanField(blank=True, null=True, "Personal 
Protection in the Home")
PPO = models.BooleanField(blank=True, null=True, "Personal 
Protection Outside the Home")

PS = models.BooleanField(blank=True, null=True, "Pistol Shooting")
RS = models.BooleanField(blank=True, null=True, "Rifle Shooting")
SSR = models.BooleanField(blank=True, null=True, "Shotgun Shell 
Reloading")

SS = models.BooleanField(blank=True, null=True, "Shotgun Shooting")
RTBVDW = models.BooleanField(blank=True, null=True, "Refuse to be 
a Victim Workshop")
RTBVO = models.BooleanField(blank=True, null=True, "Refuse to be a 
Victim Online")
NMLRA_MP = models.BooleanField(blank=True, null=True, "NMLRA 
Muzzleloading Pistol Shooting")
NMLRA_MR = models.BooleanField(blank=True, null=True, "NMLRA 
Muzzleloading Rifle Shooting")
NMLRA_MS = models.BooleanField(blank=True, null=True, "NMLRA 
Muzzleloading Shotgun Shooting")


class NRA(models.Model):
cert_type = models.ForeignKey(NRA_Certs)
name = models.CharField() # Is this necessary? Can the name be set 
to text in the boolean fields from NRA_Certs?

expiration = models.DateField(blank=True, null=True)

Does that look right or am I missing it completely. I'm not sure sure 
on the NRA_Certs all being boolean, my thinking is that whether they 
have that certification should be as simple as true/false.



Probably not. What happens if another new cert gets added to the the 
list? You will want a separate table entry for each kind of cert so when 
"Howitzer" gets added you can make that available without changing your 
models, only changing your data in the database.


afaict your new proposed class "NRA" does not add value here. Have 
certs, and have users. Should be enough.


You do have more than one piece of info for the cert model: an acronym 
and a long name or description. Those should be two fields in the model. 
Then use the many to many relation to allow multiple certs for each user 
and to allow multiple users to have the same cert.


hth

- Tom


 - Tom

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Re: ManyToManyField is this right?

[David Wagner]

Thanks for all the replies and I think I'm getting it now but it gets more
complex too.

So NRA Licenses is essentialy going to become a type of several
certifications. For instance.

NRA Instructor

   - Home Firearm Safety Instructor
   - Refuse to be a Victim Workshop
   - and many more could be selected.

CCL (or CCW in some states)

   - State in Which your Certified (possibility of multiple states)

Many more different types of licenses and certifications are going to be
added eventually so as you can see these relationships are going to get
complex. So If I'm grasping this correctly let me try some code to see if
I'm on the right track

class NRA_Certs(models.Model):
CRSO = models.BooleanField(blank=True, null=True, "Chief Range Safety
Officer")
HFS = models.BooleanField(blank=True, null=True, "Home Firearm Safety")
MCR = models.BooleanField(blank=True, null=True, "Metallic Cartridge
Reloading")
PPH = models.BooleanField(blank=True, null=True, "Personal Protection
in the Home")
PPO = models.BooleanField(blank=True, null=True, "Personal Protection
Outside the Home")
PS = models.BooleanField(blank=True, null=True, "Pistol Shooting")
RS = models.BooleanField(blank=True, null=True, "Rifle Shooting")
SSR = models.BooleanField(blank=True, null=True, "Shotgun Shell
Reloading")
SS = models.BooleanField(blank=True, null=True, "Shotgun Shooting")
RTBVDW = models.BooleanField(blank=True, null=True, "Refuse to be a
Victim Workshop")
RTBVO = models.BooleanField(blank=True, null=True, "Refuse to be a
Victim Online")
NMLRA_MP = models.BooleanField(blank=True, null=True, "NMLRA
Muzzleloading Pistol Shooting")
NMLRA_MR = models.BooleanField(blank=True, null=True, "NMLRA
Muzzleloading Rifle Shooting")
NMLRA_MS = models.BooleanField(blank=True, null=True, "NMLRA
Muzzleloading Shotgun Shooting")

class NRA(models.Model):
cert_type = models.ForeignKey(NRA_Certs)
name = models.CharField() # Is this necessary? Can the name be set to
text in the boolean fields from NRA_Certs?
expiration = models.DateField(blank=True, null=True)

Does that look right or am I missing it completely. I'm not sure sure on
the NRA_Certs all being boolean, my thinking is that whether they have that
certification should be as simple as true/false.

Thanks once again for all the help.


On Wed, Jun 27, 2012 at 7:38 AM, j_syk  wrote:> One to
One is the right type for linking a user to a profile in the Django

> world. However I think you may be mixed up on the many to many.
> How you have it setup would lead to a User being connected to multiple
> License objects, each with a nra and ccl attribute. If I didn't realize
what
> those stood for, I'd say you are right. But, I think that you might be
> thinking of it as the nra and ccl being separate licenses so in that case
> they shouldnt be the same object
>
> Here's what I'd recommend. Either put those two booleans directly on the
> profile model or make the License class more generic for any relevant
> details of a potential license. For example
>
> class License(models.Model):
> name = models.CharField(max_length=255)
> expiration = models.DateField(blank=True, null=True)
> details = models.TextField()
>
> With something like this, the User could have multiple licenses like:
> Concealed Carry 5/10/2015 and NRA no expire .
>
>
> On Wednesday, June 27, 2012 8:57:24 AM UTC-5, David Wagner wrote:
>>
>> I'm a self taught programmer who hasn't done much of anything for years
so
>> please forgive me if this question is naive. I have a hard time sometimes
>> understanding some of the lingo used by trained programmers.
>>
>> With that said, I'm having trouble wrapping my brain around the
ManyToMany
>> and Many-To-One and OneToOne concepts. I'm trying to create a
multi-select
>> option for a user profile in which a user can have multiple attributes
of a
>> certain kind. Right now I'm just working on the model. This is what I
have,
>> is it right?
>>
>> # Different licenses for instructors to select from
>> class Licenses(models.Model):
>> nra = models.BooleanField()
>> ccl = models.BooleanField()
>>
>> # User Profile Model
>> class UserProfile(models.Model):
>> user = models.OneToOneField(User)
>>
>>  #...
>>
>> licensed_instructor = models.ManyToManyField(Licenses)
>
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To u

Re: ManyToManyField is this right?

[j_syk]

One to One is the right type for linking a user to a profile in the Django 
world. However I think you may be mixed up on the many to many. 
How you have it setup would lead to a User being connected to multiple 
License objects, each with a nra and ccl attribute. If I didn't realize 
what those stood for, I'd say you are right. But, I think that you might be 
thinking of it as the nra and ccl being separate licenses so in that case 
they shouldnt be the same object

Here's what I'd recommend. Either put those two booleans directly on the 
profile model or make the License class more generic for any relevant 
details of a potential license. For example

class License(models.Model):
name = models.CharField(max_length=255)
expiration = models.DateField(blank=True, null=True)
details = models.TextField()

With something like this, the User could have multiple licenses like: 
Concealed Carry 5/10/2015 and NRA no expire .

On Wednesday, June 27, 2012 8:57:24 AM UTC-5, David Wagner wrote:
>
> I'm a self taught programmer who hasn't done much of anything for years so 
> please forgive me if this question is naive. I have a hard time sometimes 
> understanding some of the lingo used by trained programmers.
>
> With that said, I'm having trouble wrapping my brain around the ManyToMany 
> and Many-To-One and OneToOne concepts. I'm trying to create a multi-select 
> option for a user profile in which a user can have multiple attributes of a 
> certain kind. Right now I'm just working on the model. This is what I have, 
> is it right?
>
> # Different licenses for instructors to select from
> class Licenses(models.Model):
> nra = models.BooleanField()
> ccl = models.BooleanField()
>
> # User Profile Model
> class UserProfile(models.Model):
> user = models.OneToOneField(User)
> 
>  #...
>
> licensed_instructor = models.ManyToManyField(Licenses)
>

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Re: ManyToManyField is this right?

[Thomas Lockhart]

On 12-06-27 6:57 AM, David Wagner wrote:
I'm a self taught programmer who hasn't done much of anything for 
years so please forgive me if this question is naive. I have a hard 
time sometimes understanding some of the lingo used by trained 
programmers.


With that said, I'm having trouble wrapping my brain around the 
ManyToMany and Many-To-One and OneToOne concepts. I'm trying to create 
a multi-select option for a user profile in which a user can have 
multiple attributes of a certain kind. Right now I'm just working on 
the model. This is what I have, is it right?


# Different licenses for instructors to select from
class Licenses(models.Model):
nra = models.BooleanField()
ccl = models.BooleanField()
I am not familiar with the licenses you are referring to. But if "nra" 
and "ccl" are each a kind of license rather than attributes for each 
kind of license (google implies this involves gun licensing??!!) you 
will want a separate record for each license type. Something like


class Licenses(models.Model):
name = models.CharField(_("Name"), max_length=20)

then populate the license table with an entry for each license type 
"nra" and "ccl".


The "many to many" relationship you are modeling would have multiple 
license entries for multiple users and this is done by having multiple 
entries for each model rather than stacking multiple choices into a 
single model.



# User Profile Model
class UserProfile(models.Model):
user = models.OneToOneField(User)

 #...

licensed_instructor = models.ManyToManyField(Licenses)
You may want to name this field "licenses" rather than 
"licensed_instructor" for convenience and by convention.


hth

 - Tom

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Re: ManyToManyField is this right?

[Javier Guerra Giraldez]

On Wed, Jun 27, 2012 at 8:57 AM, David Wagner  wrote:
> With that said, I'm having trouble wrapping my brain around the ManyToMany
> and Many-To-One and OneToOne concepts. I'm trying to create a multi-select
> option for a user profile in which a user can have multiple attributes of a
> certain kind. Right now I'm just working on the model. This is what I have,
> is it right?

these terms are used mainly in the relational database design.  as you
might know, you can't have real arrays or structures on traditional
SQL databases, so you use those three types of 'links'.

the simplest is the 'many to one', you have one 'parent' table and a
'child' one with a field (called the "foreign key" field) where each
record keeps a copy of the primary key of a 'parent' record.  a common
example is to have 'group' and 'member' tables.  each 'member' record
has a field (sometimes called 'group_id') that tells which to which
group it belongs.  with a non-unique index on that field, it's easy to
pick all members of each group.

a slight variation is the 'one to one'.  here, the index on the
foreign key field is a 'unique' index, that forbids two records with
the same value, effectively making it impossible for a 'parent' record
to have more than one 'child'.  sometimes instead of defining a
specific foreign key field, you assign the same primary key value to
related records.

the last one is the 'many to many'.  for example, you can have users
that belong to more than one group; and of course each group has many
users.  for this, you have to define a 'link table' where each record
has two foreign key fields, one for each linked table.  sometimes the
primary key of the link table is just the composite index of these two
foreign keys.  also there can be a second composite index with the
same foreign keys in the opposite order.  Also you can choose to add
some extra fields to the link table to express some properties of the
link itself.

In Django, you handle this just by declaring fields of the appropriate
type; Django takes care of declaring the indexes, link table if
needed, and a 'reverse relationship'.  that last feature is the one
that makes the 'members' pseudo array to magically appear in 'group'
objects.

hope this helps with the theory and makes the Django docs easier to read.

-- 
Javier

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