database question

rubyonrails-talk
#1

That appears to be 5 tables, but I presume that is just a typo. What are the has and belongs to relationships?

#2

Colin Law wrote:

That appears to be 5 tables, but I presume that is just a typo.

yes, sorry for the typo.

What are the has and belongs to relationships?

Member:

has_many :memberships, :dependent => :destroy

has_many :roles, :dependent => :destroy

has_many :messages, :dependent => :destroy

Group:

has_many :mberships,:dependent => :destroy

has_many :messages, :dependent => :destroy

Role:

has_many :memberships, :dependent => :destroy

has_many :messages, :dependent => :destroy

Membership:

belongs_to :member

belongs_to :group

belongs_to :role

Message:

belongs_to :member

belongs_to :role

belongs_to :group

Are you sure? That seems incredibly complex. For example a member has many roles, but it also has many memberships, each one of which belongs to a role, so that is another set of roles a member may have.

Does a message belong to a member, a group and a role or just one of them at a time?

#3

Colin Law wrote:

Are you sure? That seems incredibly complex. For example a member has
many
roles, but it also has many memberships, each one of which belongs to a
role, so that is another set of roles a member may have.

Does a message belong to a member, a group and a role or just one of
them at
a time?

ok Colin, you're absolutely right! I'm afraid I've totally messed up my
dependencies. This is my first time to handle database relationships
through Rails models.

So, I believe this is what I want to have:

Member:
  has_many :memberships, :dependent => :destroy
  has_many :messages, :dependent => :destroy

Group:
  has_many :memberships, :dependent => :destroy
  has_many :messages, :dependent => :destroy

Role:
  has_many :memberships, :dependent => :destroy

Membership:
  belongs_to :member
  belongs_to :group
  belongs_to :role

Message:
  belongs_to :member
  belongs_to :group

A message is supposed to be written by a member, who is a part of group.
So I believe a message should belong to a member and a group.
But different members from the same group can write different messages.
And also, the same member, who is a part of different groups can write
different messages on behalf of the different groups he/she is a part
of.

I hope the relationships above reflect well my expectations that I just
described.

Thanks in advance!

#4

Am I right in thinking that the member/group combination for a message is always the same member/group that appears in a membership record? If so then would it be better to have

Membership:
has_many :messages
belongs_to :member
belongs_to :group
belongs_to :role

and

Message:
belongs_to :membership

Then you don’t need the has_many :messages for Member or Group and I think the whole thing is simpler.

#5

Colin Law wrote:

Am I right in thinking that the member/group combination for a message
is
always the same member/group that appears in a membership record? If so
then would it be better to have

Membership:
has_many :messages
belongs_to :member
belongs_to :group
belongs_to :role

and

Message:
belongs_to :membership

you are right. Then should I remove the member_id and group_id fields
from table Messages, and substitute them with a field membership_id?

:slight_smile:

#6

Colin Law wrote:

Am I right in thinking that the member/group combination for a message

is

always the same member/group that appears in a membership record? If so

then would it be better to have

Membership:

has_many :messages

belongs_to :member

belongs_to :group

belongs_to :role

and

Message:

belongs_to :membership

you are right. Then should I remove the member_id and group_id fields

from table Messages, and substitute them with a field membership_id?

Yes. The question is, of course, has any of this helped with your original question? I think it has as you can now find all messages where the membership.role_id is admin and membership.member_id is the member you want

#7

Colin Law wrote:

Yes. The question is, of course, has any of this helped with your

original

question? I think it has as you can now find all messages where the

membership.role_id is admin and membership.member_id is the member you

want

yes, we are back to the first question :slight_smile:

you are right. This is the logic, but how do I express it in Rails

syntax??

(I think) I cannot use Message.find, because the Message model doesn’t

have info about roles…

Yes it does, the relationships give it that information. If you have a message then you can say a_message.membership.role. You can also do something like
Message.find(:all, :include => :membership, :conditions => [‘membership.role_id =?’, a_role_id])

I am not guaranteeing the exact syntax, I may have made a silly mistake as I have not done much yet myself. Possibly a bit of a case of the blind leading the blind. Play about a bit and look in development.log to see the actual sql if it is not working for you.

#8

lol, that's great!

THANKS A LOT, Colin ! ! !

Have a great weekend :slight_smile:
Petar