Good module use?

ruby-talk
1

Hi,

I thought to create a module to add an instance variable representing
the connection to a database:

module MyDbModule
attr :conn
require "postgres"
pghost="localhost"
pgport=nil
pgoptions=nil
pgtty=nil
dbname="mydb"
user='mylogin’
password=‘mypassword’
@conn=PGconn.connect(pghost,pgport,pgoptions,pgtty,dbname,user,password)

end

However, this doesn’t work. The Pickaxe describes modules as a group of
calsses, methods, constants, which my modules doens’t respect (no method
defined).

Is there a way to achieve what I’m trying, and in the case that it would
be possible, is it a good approach?

An alternative would be to put all code in a method and call this method
to initialise @conn. I’m wondering if I can get rid of the call to this
method though.

Thanks.

Raph

2

Bauduin Raphael wrote:

I thought to create a module to add an instance variable representing
the connection to a database:

module MyDbModule

class <<self
attr :conn
end

  require "postgres"
  pghost="localhost"
  pgport=nil
  pgoptions=nil
  pgtty=nil
  dbname="mydb"
  user='mylogin'
  password='mypassword'

@conn=PGconn.connect(pghost,pgport,pgoptions,pgtty,dbname,user,password)

end

However, this doesn’t work. The Pickaxe describes modules as a group of
calsses, methods, constants, which my modules doens’t respect (no method
defined).

The reason is that Module#attr creates accessor methods for the
instances of the module (in reality, the instances of classes including
the module). The @conn instance variable belongs to the MyDbModule
object itself, so attr needs to be called in the scope the class of
MyDbModule. It’s due to a metaclass hierachy headache.

Is there a way to achieve what I’m trying, and in the case that it would
be possible, is it a good approach?

I’d be more inclined to use singleton, and delay connecting to the
database until it is needed.

···

–
([ Kent Dahl ]/)_ ~ [ Kent Dahl - Kent Dahl ]/~
))_student_/(( _d L b_/ Master of Science in Technology )
( __õ|õ// ) ) Industrial economics and technology management (
_/ö____/ (_engineering.discipline=Computer::Technology)

3

Bauduin Raphael a écrit :

Hi,

I thought to create a module to add an instance variable representing
the connection to a database:

module MyDbModule
attr :conn
require “postgres”
pghost=“localhost”
pgport=nil
pgoptions=nil
pgtty=nil
dbname=“mydb”
user=‘mylogin’
password=‘mypassword’

@conn=PGconn.connect(pghost,pgport,pgoptions,pgtty,dbname,user,password)

end
To use a module variable through “MyModule.my_var”, just prefix the
variable name in the module with “self.” or “@@”.

Lio

4

Hi --

Bauduin Raphael a écrit :

> Hi,
>
> I thought to create a module to add an instance variable representing
> the connection to a database:
>
> module MyDbModule
> attr :conn
> require "postgres"
> pghost="localhost"
> pgport=nil
> pgoptions=nil
> pgtty=nil
> dbname="mydb"
> user='mylogin'
> password='mypassword'
>
> @conn=PGconn.connect(pghost,pgport,pgoptions,pgtty,dbname,user,password)
>
> end
To use a module variable through "MyModule.my_var", just prefix the
variable name in the module with "self." or "@@".

Not exactly; @@var is a class variable; self.var is a method call. If
you want an association between them, you have to write appropriate
methods:

  class X
    def self.my_var
      @@my_var
    end
    # etc.
  end

David

···

On Sun, 4 Apr 2004, Lionel Thiry wrote:

--
David A. Black
dblack@wobblini.net

5

http://segment7.net/projects/ruby/snippets/database.rb

Is an example of this approach.

···

Kent Dahl (kentda+news@stud.ntnu.no) wrote:

Is there a way to achieve what I’m trying, and in the case that it would
be possible, is it a good approach?

I’d be more inclined to use singleton, and delay connecting to the
database until it is needed.

–
Eric Hodel - drbrain@segment7.net - http://segment7.net
All messages signed with fingerprint:
FEC2 57F1 D465 EB15 5D6E 7C11 332A 551C 796C 9F04

6

David A. Black a écrit :

···

On Sun, 4 Apr 2004, Lionel Thiry wrote:

Bauduin Raphael a écrit :

Hi,

I thought to create a module to add an instance variable representing
the connection to a database:

module MyDbModule
attr :conn
require “postgres”
pghost=“localhost”
pgport=nil
pgoptions=nil
pgtty=nil
dbname=“mydb”
user=‘mylogin’
password=‘mypassword’

@conn=PGconn.connect(pghost,pgport,pgoptions,pgtty,dbname,user,password)

end

To use a module variable through “MyModule.my_var”, just prefix the
variable name in the module with “self.” or “@@”.
Not exactly; @@var is a class variable; self.var is a method call. If
you want an association between them, you have to write appropriate
methods:
Oops, wrote my advice too fast! :wink:
Well, I’ve found how I could write such a mistake… Learning from
mistakes is always a good thing, isn’t it? (but teaching them is not,
sorry!)

Lio