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This is my table
I want to add a constraint to the database so that a couple of user_id will be rejected if its symetric already exists.
For example, if I already have the couple
(42, 17) in the db, I don't want to be able to save
I'm using rails with postgresql.
You can do this in the database using an index on expressions:
create unique index unq_friendship_1_2 on (least(user_id1, user_id2), greatest(user_id1, user_id2));
I do not know how you express this in rails.
ruby-on-rails, Unicity on two columns asymmetrically. 发表于 2016-05-09 10:45:53. 活跃于 2016-05-09 11:09:35. 查看51 次. ruby-on-rails postgresql constraints unique-� To retrieve unique data based on multiple columns, you just need to specify the column list in the SELECT clause as follows: SELECT DISTINCT column_1, column_2, column_3 FROM table_name; In this syntax, the combination of values in the column_1 , column_2 , and column_3 are used to determine the uniqueness of the data.
You can do this with a combination of a normal unique constraint and a check constraint, if you are able to implement the application in a way, that it stores the smaller value in the field user_id_1 and the bigger value in the field user_id_2.
Define a unique constraint on (user_id_1, user_id_2) and additionally a check constraint, that (user_id_1 <= user_id_2) to verify that your application behaves in the way defined above.
In Oracle this would be
ALTER TABLE tablename ADD CONSTRAINT cname1 UNIQUE (user_id_1, user_id_2) ALTER TABLE tablename ADD CONSTRAINT cname2 CHECK (user_id_1 <= user_id_2)
If your application is not able to insert the values sorted as described above, you still can use e.g. triggers, but this is more DB-vendor specific.
SQLite UNIQUE Constraint, Only at the table level, you can define a UNIQUE constraint across multiple columns. The following shows how to define a UNIQUE constraint for a column at the� The P.E. June/Thomas Ireland home is a 2 ½ story brick duplex in the Colonial Revival style. It was constructed in 1924. The main facade is dominated by two rectangular porches supported on two columns, surrounding two single door entries. The doors are flanked by large fixed pane windows. A small window is located between the doors.
You can do it like this in Rails.
In your rails model
friendship.rb, add the following
validates_uniqueness_of :user_id1, :scope => :user_id2.
This guarantees that there will be no two identical couples of
However, @potashin has explained that this method is not totally safe because if two simultaneous requests are sent they may be accepted and your rows would be duplicated. His solution is to also add the following migration to your Friendships model:
class AddUniqueConstraints < ActiveRecord::Migration def change add_index :friendships, [:user_id1, :user_id2], unique: true end end
Unicity distance | Crypto Wiki, D is defined as the plaintext redundancy in bits per character. Now an alphabet of 32 characters can carry 5 bits of information per character (as 32 = 25). In� Unicity on two columns asymmetrically. Posted: 09 May 2016 04:09 AM PDT. This is my table Friendship(user_id1:integer, user_id2:integer) I want to add a constraint to
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