I have a bunch of customers that need to be sorted by the time they were "completed" according to our system. I can do it like this and it works ok:
customers = Customer.where(state: 'review').joins(:audits)
customers.sort_by { |c| c.onboarding_finished_at.to_i }
or...it did until it got onto our staging environment where we have enough customers with enough audits that it caused some major performance issues. Now i'm searching for a way to sort customers by the created_at attribute of the last state transition, but my query-foo is woefully inadequate.
this is a simplified version of the audits data associated with a customer. Note the audited_changes column which is jsonb:
irb(main):032:0> customers.last.audits.last
=> #<Audited::Audit id: 642691, auditable_id: 45517, action: "update", audited_changes: {"state"=>[0, 1]}, created_at: "2020-08-13 08:59:00">
I can access the audits but i can't for the life of me figure out how to get the last transition to state "1" (cause there could be multiple) and then pluck the created_at of that and then sort customers by that created_at value.
Any push in the right direction is very much appreciated!
You can do an inner joins on the table and run a query to check second element of state array (assuming that's the structure for all the records) is 1.
Customer.includes(:audits).where("audited_changes->'$.state[1]' = 1").reorder("audits.created_at asc")
Let me know if that's what you're looking for
Model.first doesnot retrive first record from table. Instead it retrives any random record from table.
eg:
Merchant.first
Query
SELECT "merchants".* FROM "merchants" LIMIT 1
=> <Merchant id: 6, merchant_name: "Bestylish", description: "", description_html: "" >
Instead the query should be
SELECT "merchants".* FROM "merchants" ORDER BY "merchants"."id" ASC LIMIT 1;
Why it doesnot retrive the first record
Model.first will use the default sorting of your database.
For example. In Postgresql default sorting is not necessarily an id.
This seems to be default behaviour with Postgres, as some active-record versions do not add a default ordering to the query for first, while adding one for last.
https://github.com/rails/rails/issues/9885
PostgreSQL does not by default apply a sort, which is generally a good thing for performance.
So in this context "first" means "the first row returned", not "the first row when ordered by some meaningless key value".
Curiously "last" does seem to order by id.
It is defined here, in Rails 4, to order by primary key if no other order conditions are specified.
In Rails 3.2.11, it is as such:
def find_first
if loaded?
#records.first
else
#first ||= limit(1).to_a[0]
end
end
Without the order method, which will just apply the limit and then leave the ordering up to your database.
You need to apply the ordering yourself. Try calling Merchant.order('id ASC').first
It may be possible to automate this using default scopes in your model but I'm not sure about that.
I want to fetch the last 5 records where transfer = something. Currently I'm using: Model.limit(5).order('id desc') but that shows the last 5 of Call.all. How can I express this by adding a where clause or specifying a particular value as part of the query?
Simply
Model.where(:transfer => 'something').order('id desc').limit(5)
But it might make more sense to order by created_at or updated_at instead of id.
In my Rails app I've run into an issue a couple times that I'd like to know how other people solve:
I have certain records where a value is optional, so some records have a value and some are null for that column.
If I order by that column on some databases the nulls sort first and on some databases the nulls sort last.
For instance, I have Photos which may or may not belong to a Collection, ie there are some Photos where collection_id=nil and some where collection_id=1 etc.
If I do Photo.order('collection_id desc) then on SQLite I get the nulls last but on PostgreSQL I get the nulls first.
Is there a nice, standard Rails way to handle this and get consistent performance across any database?
I'm no expert at SQL, but why not just sort by if something is null first then sort by how you wanted to sort it.
Photo.order('collection_id IS NULL, collection_id DESC') # Null's last
Photo.order('collection_id IS NOT NULL, collection_id DESC') # Null's first
If you are only using PostgreSQL, you can also do this
Photo.order('collection_id DESC NULLS LAST') #Null's Last
Photo.order('collection_id DESC NULLS FIRST') #Null's First
If you want something universal (like you're using the same query across several databases, you can use (courtesy of #philT)
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
Even though it's 2017 now, there is still yet to be a consensus on whether NULLs should take precedence. Without you being explicit about it, your results are going to vary depending on the DBMS.
The standard doesn't specify how NULLs should be ordered in comparison with non-NULL values, except that any two NULLs are to be considered equally ordered, and that NULLs should sort either above or below all non-NULL values.
source, comparison of most DBMSs
To illustrate the problem, I compiled a list of a few most popular cases when it comes to Rails development:
PostgreSQL
NULLs have the highest value.
By default, null values sort as if larger than any non-null value.
source: PostgreSQL documentation
MySQL
NULLs have the lowest value.
When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC.
source: MySQL documentation
SQLite
NULLs have the lowest value.
A row with a NULL value is higher than rows with regular values in ascending order, and it is reversed for descending order.
source
Solution
Unfortunately, Rails itself doesn't provide a solution for it yet.
PostgreSQL specific
For PostgreSQL you could quite intuitively use:
Photo.order('collection_id DESC NULLS LAST') # NULLs come last
MySQL specific
For MySQL, you could put the minus sign upfront, yet this feature seems to be undocumented. Appears to work not only with numerical values, but with dates as well.
Photo.order('-collection_id DESC') # NULLs come last
PostgreSQL and MySQL specific
To cover both of them, this appears to work:
Photo.order('collection_id IS NULL, collection_id DESC') # NULLs come last
Still, this one does not work in SQLite.
Universal solution
To provide cross-support for all DBMSs you'd have to write a query using CASE, already suggested by #PhilIT:
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
which translates to first sorting each of the records first by CASE results (by default ascending order, which means NULL values will be the last ones), second by calculation_id.
Photo.order('collection_id DESC NULLS LAST')
I know this is an old one but I just found this snippet and it works for me.
Put minus sign in front of column_name and reverse the order direction. It works on mysql. More details
Product.order('something_date ASC') # NULLS came first
Product.order('-something_date DESC') # NULLS came last
Bit late to the show but there is a generic SQL way to do it. As usual, CASE to the rescue.
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
The easiest way is to use:
.order('name nulls first')
For posterity's sake, I wanted to highlight an ActiveRecord error relating to NULLS FIRST.
If you try to call:
Model.scope_with_nulls_first.last
Rails will attempt to call reverse_order.first, and reverse_order is not compatible with NULLS LAST, as it tries to generate the invalid SQL:
PG::SyntaxError: ERROR: syntax error at or near "DESC"
LINE 1: ...dents" ORDER BY table_column DESC NULLS LAST DESC LIMIT...
This was referenced a few years ago in some still-open Rails issues (one, two, three). I was able to work around it by doing the following:
scope :nulls_first, -> { order("table_column IS NOT NULL") }
scope :meaningfully_ordered, -> { nulls_first.order("table_column ASC") }
It appears that by chaining the two orders together, valid SQL gets generated:
Model Load (12.0ms) SELECT "models".* FROM "models" ORDER BY table_column IS NULL DESC, table_column ASC LIMIT 1
The only downside is that this chaining has to be done for each scope.
Rails 6.1 adds nulls_first and nulls_last methods to Arel for PostgreSQL.
Example:
User.order(User.arel_table[:login_count].desc.nulls_last)
Source: https://www.bigbinary.com/blog/rails-6-1-adds-nulls-first-and-nulls-last-to-arel
Here are some Rails 6 solutions.
The answer by #Adam Sibik is a great summary about the difference between various database systems.
Unfortunately, though, some of the presented solutions, including "Universal solution" and "PostgreSQL and MySQL specific", would not work any more with Rails 6 (ActiveRecord 6) as a result of its changed specification of order() not accepting some raw SQLs (I confirm the "PostgreSQL specific" solution still works as of Rails 6.1.4). For the background of this change, see, for example,
"Updates for SQL Injection in Rails 6.1" by Justin.
To circumvent the problem, you can wrap around the SQL statements with Arel.sql as follows, where NULLs come last, providing you are 100% sure the SQL statements you give are safe.
Photo.order(Arel.sql('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id'))
Just for reference, if you want to sort by a Boolean column (is_ok, as an example) in the order of [TRUE, FALSE, NULL] regardless of the database systems, either of these should work:
Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 ELSE 0 END, is_ok DESC'))
Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 WHEN is_ok IS TRUE THEN -1 ELSE 0 END'))
(n.b., SQLite does not have the Boolean type and so the former may be safer arguably, though it should not matter because Rails should guarantee the value is either 0 or 1 (or NULL).)
In my case I needed sort lines by start and end date by ASC, but in few cases end_date was null and that lines should be in above, I used
#invoice.invoice_lines.order('start_date ASC, end_date ASC NULLS FIRST')
Adding arrays together will preserve order:
#nonull = Photo.where("collection_id is not null").order("collection_id desc")
#yesnull = Photo.where("collection_id is null")
#wanted = #nonull+#yesnull
http://www.ruby-doc.org/core/classes/Array.html#M000271
It seems like you'd have to do it in Ruby if you want consistent results across database types, as the database itself interprets whether or not the NULLS go at the front or end of the list.
Photo.all.sort {|a, b| a.collection_id.to_i <=> b.collection_id.to_i}
But that is not very efficient.
I was wondering if there is a way to find the newest record in a table in rails3?
Given a Post model, you could do #post = Post.order("created_at").last
(The reason I didn't just do a #post = Post.last is because that always defaults to sort by your primary key (usually id). Most of the time this is fine, but I'm sure there's a scenario where that could cause problems (e.g. setting custom IDs on records, database changes which affect the primary key sequencing/autonumbering, etc.). Sorting by the created_at timestamp ensures you are really getting the most recent record).
While dmarkow's answer is technically correct, you'll need to make an index on created_at or risk an increasingly slow query as your database grows.
If you know that your "id" column is an auto-increment primary key (which it likely is), then just use it since it is an index by definition.
Also, unless AREL is optimized to select only one record in a find(:last), you run the risk of making it select ALL records, then return you just the last one by using the "last()" method. More efficient is to limit the results to one:
MyModel.last(:order => "id asc", :limit => 1)
or
MyModel.first(:order => "id desc", :limit => 1)
you may run into ambiguity issues using created_at on a sufficiently high-traffic table.
eg. try:
INSERT INTO table (created_at) VALUES ( NOW() );
INSERT INTO table (created_at) VALUES ( NOW() );
..has the potential to have the same created_at, which only has 1 second of resolution. a sort would return them in no particular order.
you may be better off storing a microtime value and sorting on that.
Yes, you can use the method .last
So if your model is called Post then:
>> Post.last
=> #<Post ...>
Try, for a model named ModelName:
record = ModelName.last