I have a Rails 3 app which keeps high scores. I'm hosting it on Heroku which uses postgresql as the db.
I need to extract the top scores from the scores table. The table has columns score and user_id. It was working in mysql with the following:
Score.order('score DESC').group('user_id').limit(25)
This ranks each user's top score.
When I put the app on Heroku, I get the following psql error PGError: ERROR: column "scores.id" must appear in the GROUP BY clause or be used in an aggregate function
I've read around but haven't found a clear answer. What is the most optimal way to recreate the above query to work with PostgreSQL?
Thanks!
Tim
That means your select query is selecting the "id" column but not including it in the group by clause. I'm not familiar with rails but could it be selecting * or all columns?
Could you do
select id, max(score) group by id;
Related
I got the data by 'MODEL.all' command in rails console
I want to put the column 'cgi_name' in the 3rd position when I run MODEL.all in the rails console
I use the postgres for my DB
How to get it ?
To answer your question directly, you'll have to move the columns at DB level
Currently, I only know MYSQL to support this functionality:
ALTER TABLE Employees CHANGE COLUMN empName empName VARCHAR(50) AFTER department;
Postgres, to my knowledge, does not support this functionality:
Many people new to postgresql often ask if it has support for altering
column positions within a table. Currently it does not; if you want to
change column positions, you must either recreate the table, or add
new columns and move data
In the view, you'll have to either manually display the columns, or create a helper method to cycle through them in an order of your choosing
Simple answer is YOU CANNOT
There is no way to re-order the column names to be displayed when you select using Model.all.
Otherwise, you can re-order this by selecting each column in the order you want.
Model.select("column1, column2, cgi_name, column4 etc..")
Hope it helps :)
Environment: Rails 3.2.13 with SQLite in development and PostgreSQL in production
Using Active Record, I'm trying to get a list of unique first letters from all first names in the DB. For example, if I have names (Alex, Billy, Bob, Dave, Jack, James, Jen, Sam, Steve, Stewart, Victor) I'm trying to get the letters (A,B,D,J,S,V) from the DB.
I've been able to go into the Rails DB Console and query SQLite and get what I want using:
SELECT DISTINCT SUBSTR(last_name,1,1) FROM people ORDER BY last_name;
But haven't been able to get it to work within Rails.
you can use, for instance
User.all.map{|u| u.name[0]}.uniq
but this is in memory. If you need to do it in SQL, you can use find_by_sql
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
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}.
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.
I'd like to use the rank() function PostgreSQL on one of my columns.
Character.select("*, rank() OVER (ORDER BY points DESC)")
But since I don't have a rank column in my table rails doesn't include it with the query. What would be the correct way to get the rank included in my ActiveRecord object?
try this:
Character.find_by_sql("SELECT *, rank() OVER (ORDER BY points DESC) FROM characters")
it should return you Character objects with a rank attribute, as documented here. However, this may not be database-agnostic and tends to get messy if you pass around the objects.
another (expensive) solution is to add a rank column to your table, and have a callback recalculate all records' rank using .order whenever a record is saved or destroyed.
edit :
another idea suitable for single-record queries can ben seen here
I am using Rails 3 and postgresql. I have the following genres: rock, ambience, alternative, house.
I also have two users registered. One has rock and the other house, as their genres. I need to return rock and house genre objects.
I found two ways to do this. One is using group:
Genre.group('genres.id, genres.name, genres.cached_slug, genres.created_at, genres.updated_at').joins(:user).all
And the other using DISTINCT:
Genre.select('DISTINCT(genres.name), genres.cached_slug').joins(:user)
Both return the same desired results. But which one is better performance wise? Using group() looks messy since I have to indicate all the fields in the Genre table otherwise I'll get errors as such:
ActiveRecord::StatementInvalid: PGError: ERROR: column "genres.id" must appear in the GROUP BY clause or be used in an aggregate function
: SELECT genres.id FROM "genres" INNER JOIN "users" ON "users"."genre_id" = "genres"."id" GROUP BY genres.name
A DISTINCT and GROUP BY usually generate the same query plan, so performance should be the same across both query constructs.
Since you're not using any aggregate functions, you should use the one that makes the most sense in your situation, which I believe is this one:
Genre.select('DISTINCT(genres.name), genres.cached_slug').joins(:user)
This will be more clear when trying to read your code later and remember what you did here and, as you pointed out, is much less messy.
Update
It depends on the version of Postgresql you are using. Using versions < 8.4, GROUP BY is faster. With version 8.4 and later, they are the same.