How I do different has_and_belongs_to_many for the same model?

Rails
1

Hi,

I have a team which has and belongs to many players.

class Team < ActiveRecord::Base
has_and_belongs_to_many :players
end

class Player < ActiveRecord::Base
has_and_belongs_to_many :teams
end

What I’d like to do is add bench players to my team. The bench will
have many players and many players can be on the bench.

I tried to do it like this:

class Team < ActiveRecord::Base
has_and_belongs_to_many :players
has_and_belongs_to_many :bench_players, :class => ‘Player’
end

I made the association table like this:

class TeamsBenchPlayers < ActiveRecord::Migration
def self.up
create_table :teams_bench_players, :id => false do |t|
t.column :team_id, :integer, :null => false
t.column :player_id, :integer, :null => false
end
add_index :teams_bench_players, [:team_id]
add_index :teams_bench_players, [:player_id]
end
def self.down
drop_table :teams_bench_players
end
end

However when I do this:

team.bench_players << player

It saves it in the wrong table, i.e. the team_players table. What am
I doing wrong?

Thanks,
Frank

2

On Mar 2, 5:47 am, Frank K. [email protected] wrote:

However when I do this:

team.bench_players << player

It saves it in the wrong table, i.e. the team_players table. What am
I doing wrong?

You need to tell activerecord to use a different join table (with
the :join_table) option

Fred

3

Some might argue that being on the bench or not is simply an attribute
of a player… benched or playing, the player is still on the team.

4

Hi Ar,
The attribute idea makes a lot of sense. If I did it that way then I
could use the same join table. However I would have to use specify
custom SQL for how to select from it, right? Or is there a better
way?
-Frank

On Tue, Mar 2, 2010 at 5:58 AM, Ar Chron [email protected] wrote:

–
Frank K.

5

Thanks Frederick, I forgot about that option.

On Tue, Mar 2, 2010 at 3:00 AM, Frederick C.
[email protected] wrote:

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Frank K.

6

Frank K. wrote:

Hi Ar,
The attribute idea makes a lot of sense. If I did it that way then I
could use the same join table. However I would have to use specify
custom SQL for how to select from it, right? Or is there a better
way?

You could just have “benched” be a boolean field in the database. If
it’s true, the player’s on the bench. If it’s false, he’s on the field.
Then you can do this:

player = Player.find_by_last_name “Ronaldo”
player.benched = true # Be a team player, Ronaldo!

7

Not sure if has_and_belongs_to_many is exactly right (can a player be
on more than one team?) but one option is to add :conditions to the
declarations:

has_and_belongs_to_many :benched_players, :join_table =>
‘teams_players’, :association_foreign_key => ‘player_id’, :class_name
=> ‘Player’, :conditions => { :benched => true }

(not sure if this is 100% of what you need, but you get the idea)

On the other hand, if a player really can only be on one team, you’ll
have a much better time if you switch back to a plain has_many. On the
other hand, if you’ve got a situation where players can be on
multiple teams, it’s possible that they could be “benched” only on
some of them; in that case, you’ll want a “decorated join model”, with
has_many :through.

–Matt J.

8

The attribute (Matt J. calls it a “condition”) of being on the bench
or
not on the bench is a property of the player not the team.

A team does not belong to the players. Players make up the team.

Consider the deletion anomaly. If a Player is removed, does the team
cease
to exist?

However, If a team is removed, then none of the players can
theoretically
belong to that team.

On 2 March 2010 13:51, Matt J. [email protected] wrote:

On the other hand, if a player really can only be on one team, you’ll

Hi Ar,
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[email protected]

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Let’s talk.
(IBM Partner)

9

That makes a lot of sense Ar. Thanks!

On Tue, Mar 2, 2010 at 6:22 AM, Aldric G. [email protected]
wrote:

–
Frank K.
http://betweengo.com/

10

Hi Ar,
Actually I realized this won’t work because players can be on multiple
teams and can be benched on one team but not the other. Therefore it
can’t be an attribute of the player, it has to be an attribute of the
team.
Looks like I’ll have to go back to my old idea.
-Frank

On Tue, Mar 2, 2010 at 6:22 AM, Aldric G. [email protected]
wrote:

–
Frank K.

11

Frederick,
That was the solution.
Thanks!
-Frank

On Tue, Mar 2, 2010 at 6:01 AM, Frank K. [email protected] wrote:

team.bench_players << player
Frank

–
Frank K.
http://betweengo.com/

–
Frank K.