2 column table, ignore duplicates on mass insert postgresql - ruby-on-rails

Question

I have a Join table in Rails which is just a 2 column table with ids.
In order to mass insert into this table, I use
ActiveRecord::Base.connection.execute("INSERT INTO myjointable (first_id,second_id) VALUES #{values})
Unfortunately this gives me errors when there are duplicates. I don't need to update any values, simply move on to the next insert if a duplicate exists.
How would I do this?
As an fyi I have searched stackoverflow and most the answers are a bit advanced for me to understand. I've also checked the postgresql documents and played around in the rails console but still to no avail. I can't figure this one out so i'm hoping someone else can help tell me what I'm doing wrong.
The closest statement I've tried is:
INSERT INTO myjointable (first_id,second_id) SELECT 1,2
WHERE NOT EXISTS (
SELECT first_id FROM myjointable
WHERE first_id = 1 AND second_id IN (...))
Part of the problem with this statement is that I am only inserting 1 value at a time whereas I want a statement that mass inserts. Also the second_id IN (...) section of the statement can include up to 100 different values so I'm not sure how slow that will be.
Note that for the most part there should not be many duplicates so I am not sure if mass inserting to a temporary table and finding distinct values is a good idea.
Edit to add context:
The reason I need a mass insert is because I have a many to many relationship between 2 models where 1 of the models is never populated by a form. I have stocks, and stock price histories. The stock price histories are never created in a form, but rather mass inserted themselves by pulling the data from YahooFinance with their yahoo finance API. I use the activerecord-import gem to mass insert for stock price histories (i.e. Model.import columns,values) but I can't type jointable.import columns,values because I get the jointable is an undefined local variable

Answer 1

I ended up using the WITH clause to select my values and give it a name. Then I inserted those values and used WHERE NOT EXISTS to effectively skip any items that are already in my database.
So far it looks like it is working...
WITH withqueryname(first_id,second_id) AS (VALUES(1,2),(3,4),(5,6)...etc)
INSERT INTO jointablename (first_id,second_id)
SELECT * FROM withqueryname
WHERE NOT EXISTS(
SELECT first_id FROM jointablename WHERE
first_id = 1 AND
second_id IN (1,2,3,4,5,6..etc))
You can interchange the Values with a variable. Mine was VALUES#{values}
You can also interchange the second_id IN with a variable. Mine was second_id IN #{variable}.

Answer 2

Here's how I'd tackle it: Create a temp table and populate it with your new values. Then lock the old join values table to prevent concurrent modification (important) and insert all value pairs that appear in the new table but not the old one.
One way to do this is by doing a left outer join of the old values onto the new ones and filtering for rows where the old join table values are null. Another approach is to use an EXISTS subquery. The two are highly likely to result in the same query plan once the query optimiser is done with them anyway.
Example, untested (since you didn't provide an SQLFiddle or sample data) but should work:
BEGIN;
CREATE TEMPORARY TABLE newjoinvalues(
first_id integer,
second_id integer,
primary key(first_id,second_id)
);
-- Now populate `newjoinvalues` with multi-valued inserts or COPY
COPY newjoinvalues(first_id, second_id) FROM stdin;
LOCK TABLE myjoinvalues IN EXCLUSIVE MODE;
INSERT INTO myjoinvalues
SELECT n.first_id, n.second_id
FROM newjoinvalues n
LEFT OUTER JOIN myjoinvalues m ON (n.first_id = m.first_id AND n.second_id = m.second_id)
WHERE m.first_id IS NULL AND m.second_id IS NULL;
COMMIT;
This won't update existing values, but you can do that fairly easily too by using with a second query that does an UPDATE ... FROM while still holding the write table lock.
Note that the lock mode specified above will not block SELECTs, only writes like INSERT, UPDATE and DELETE, so queries can continue to be made to the table while the process is ongoing, you just can't update it.
If you can't accept that an alternative is to run the update in SERIALIZABLE isolation (only works properly for this purpose in Pg 9.1 and above). This will result in the query failing whenever a concurrent write occurs so you have to be prepared to retry it over and over and over again. For that reason it's likely to be better to just live with locking the table for a while.