Ruby – How to update or insert on Sequel dataset

rubysequelupsert

I just started using Sequel in a really small Sinatra app. Since I've got only one DB table, I don't need to use models.

I want to update a record if it exists or insert a new record if it does not. I came up with the following solution:

  rec = $nums.where(:number => n, :type => t)
  if $nums.select(1).where(rec.exists)
    rec.update(:counter => :counter + 1)
  else
    $nums.insert(:number => n, :counter => 1, :type => t)
  end

Where $nums is DB[:numbers] dataset.

I believe that this way isn't the most elegant implementation of "update or insert" behavior.

How should it be done?

Best Answer

You should probably not check before updating/inserting; because:

This is an extra db call.
This could introduce a race condition.

What you should do instead is to test the return value of update:

rec = $nums.where(:number => n, :type => t)
if 1 != rec.update(:counter => :counter + 1)
  $nums.insert(:number => n, :counter => 1, :type => t)
end

Related Solutions

Sql – Solutions for INSERT OR UPDATE on SQL Server

don't forget about transactions. Performance is good, but simple (IF EXISTS..) approach is very dangerous.
When multiple threads will try to perform Insert-or-update you can easily get primary key violation.

Solutions provided by @Beau Crawford & @Esteban show general idea but error-prone.

To avoid deadlocks and PK violations you can use something like this:

begin tran
if exists (select * from table with (updlock,serializable) where key = @key)
begin
   update table set ...
   where key = @key
end
else
begin
   insert into table (key, ...)
   values (@key, ...)
end
commit tran

begin tran
   update table with (serializable) set ...
   where key = @key

   if @@rowcount = 0
   begin
      insert into table (key, ...) values (@key,..)
   end
commit tran

Sql – Oracle: how to UPSERT (update or insert into a table?)

The MERGE statement merges data between two tables. Using DUAL allows us to use this command. Note that this is not protected against concurrent access.

create or replace
procedure ups(xa number)
as
begin
    merge into mergetest m using dual on (a = xa)
         when not matched then insert (a,b) values (xa,1)
             when matched then update set b = b+1;
end ups;
/
drop table mergetest;
create table mergetest(a number, b number);
call ups(10);
call ups(10);
call ups(20);
select * from mergetest;

A                      B
---------------------- ----------------------
10                     2
20                     1

Best Answer

Related Solutions

Sql – Solutions for INSERT OR UPDATE on SQL Server

Sql – Oracle: how to UPSERT (update or insert into a table?)

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