Currently I can make the straight-up SQL query on my DB:
SELECT MAX(bar) FROM table_name
And it returns with the max value in that table. When I make what I consider to be an equivalent call in Rails, however, it does not work. I am calling:
Bar.all(:select => "Max(bar)")
This simply returns with:
[#<Bar >]
In the column I'm calling on is a series of identifying numbers, I'm looking for the largest one. Is there some other way of accessing this in Rails?
Assuming your model name is Bar and it has a column named bar, this should work:
Bar.maximum(:bar)
See the excellent Rails Guides section on Calculations :: Maximum for more info.
one more way
Bar.select("Max(bar) as max_bar").first.max_bar
Related
Let's say I have the following 'items' table in my PostgreSQL database:
id
item
value
1
a
10
2
b
20
3
c
30
For some reason I can't control I need to run the following query:
select max(value) from items;
which will return 30 as the result.
At this point, I know that I can find the record that contains that value using simple select statements, etc. That's not the actual problem.
My real questions are:
Does PostgreSQL know (behind the scenes) what's is the ID of that
record, although the query shows only the max value of the column
'value'?
If yes, can I have access to that information and,
therefore, get the ID and other data from the found record?
I'm not allowed to create indexes and sequences, or change way the max value is retrieved. That's a given. I need to work from that point onward and find a solution (which I have, actually, from regular query work).
I'm just guessing that the database knows in which record that information (30) is and that I could have access to it.
I've been searching for an answer for a couple of hours but wasn't able to find anything.
What am I missing? Any ideas?
Note: postgres (PostgreSQL) 12.5 (Ubuntu 12.5-0ubuntu0.20.10.1)
You can simply extract the whole record that contains max(value) w/o bothering about Postgres internals like this:
select id, item, "value"
from items
order by "value" desc
limit 1;
I do not think that using undocumented "behind the scenes" ways is a good idea at all. The planner is smart enough to do exactly what you need w/o extra work.
I am using Rails 4 and I need to use find_by_sql in my active record model.Now I am facing two serious problems. First one is that it does not give me a particluar data whether it is giving #Employee:0x0000000b2a1718 as result. My model name is Employee and tbale name is employees. I am using pg.
Please tell me is there any solution.
Second problem is that how can I write any rails variable with in the sql query used in find_by_sql. For example I want to execute find_by_sql("select firstname from employee where id=#var"), where #var is a ruby variable.
The actual query I need to execute is select firstname from employee where comapact_string like %#var% using find_by_sql.
There's degrees of customization when making a query. The simplest form is where you can use the built-in finders:
Employee.where(id: #var).pluck(:first name)
That will do a direct match, and if one's found, give you the first_name column value. No model is produced with pluck.
If you want to do an approximate match with LIKE you write out the WHERE clause more formally:
Employee.where('id LIKE ?', "%{#var}%").pluck(:first_name)
It's rare you need to write out an entire query with find_by_sql, but if you do you must be extremely cautious about what data you put in the query. It's strongly recommended to use placeholder values whenever possible, and if you absolutely must bypass this, escape everything no matter the source.
I am trying to learn rails [by following the SAAS course in coursera] and working with simple Movie table using ActiveRecord.
I want to display all movies with title sorted. I would like it to be sorted case insensitively.
I tried doing it this way:
Movie.all(:conditions => ["lower(title) = ?", title.downcase],:order => "title DESC")
=>undefined local variable or method `title' for #<MoviesController:0xb4da9a8>
I think it doesnt recognise lower(title) .
Is this the best way to achieve case insesisitve sort ?
Thanks!
Use where and not all
Movie.where("lower(title) = ?", title.downcase).order("title DESC")
Don't really understand the sort though. Here you'll get all movies with lower title equalling to title.downcase. Everything is equal, how could you sort it by title desc ?
To sort reverse-alphabetically all movies by lowercase title :
Movie.order("lower(title) DESC").all
You have to do this:
Movie.order("lower(title) DESC").all
A more robust solution is to use arel nodes. I'd recommend defining a couple scopes on the Movie model:
scope :order_by_title, -> {
order(arel_table['title'].lower.desc)
}
scope :for_title, (title)-> {
where(arel_table['title'].lower.eq title.downcase)
}
and then call Movie.for_title(title).order_by_title
The advantage over other answers listed is that .for_title and .order_by_title won't break if you alias the title column or join to another table with a title column, and they are sql escaped.
Like rickypai mentioned, if you don't have an index on the column, the database will be slow. However, it's bad (normal) form to copy your data and apply a transform to another column, because then one column can become out of sync with the other. Unfortunately, earlier versions of mysql didn't allow for many alternatives other than triggers. After 5.7.5 you can use virtual generated columns to do this. Then in case insensitive cases you just use the generated column (which actually makes the ruby more straight forward).
Postgres has a bit more flexibility in this regard, and will let you make indexes on functions without having to reference a special column, or you can make the column a case insensitive column.
Having MySQL perform upper or lower case operation each time is quite expensive.
What I recommend is having a title column and a title_lower column. This way, you can easily display and sort with case insensitivity on the title_lower column without having MySQL perform upper or lower each time you sort.
Remember to index both or at least title_lower.
I have photos which have_many comments.
I want to select whatever photos have recent comments and display those photos in a kind of "timeline" where the most recently commented photo is at the top and other photos fall below.
I tried this, and it worked on SQLite:
#photos = Photo.select('DISTINCT photos.*').joins(:comments).order('comments.created_at DESC')
However testing on PostgreSQL raises this error:
PGError: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
\n: SELECT DISTINCT photos.* FROM \"photos\" INNER JOIN \"comments\" ON \...
So, the problem is, I'm selecting Photos but ordering by recency of comments... and Postgre doesn't like that.
Can anyone suggest either:
A: How I can fix this query...
or
B: A different way to retrieve photos by the recency of their comments?
The important reason I'm doing it this way instead of through the comments model is I want to show each photo once with any recent comments beside it, not show each comment by itself with the same photos appearing multiple times.
Thanks!
Check out the :touch parameter of of the belongs_to association:
:touch
If true, the associated object will be
touched (the updated_at/on attributes
set to now) when this record is either
saved or destroyed. If you specify a
symbol, that attribute will be updated
with the current time instead of the
updated_at/on attribute.
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to
In your Comment model, therefore, you would have:
belongs_to :photo, :touch => :comments_updated_at
Now, in order to create a time line of photos with recently updated comments all you need to do is:
Photo.order('comments_updated_at DESC').all
Just be sure to add the "comments_updated_at" datetime field to your Photo model.
Make sense?
Just for the future readers of this question, the real answer to your SQL issue in SQlite vs Postgresql is that in the SQL "standard", every selected column needs to be in the GROUP BY or be an aggregate function.
https://www.techonthenet.com/sql/group_by.php (or whatever SQL ref you want to take a look at)
Your SQLite query used SELECT * instead of specific columns. That would have blown up with a similar error on most databases like Postgresql (MySQL, Maria, probably MSSQL Server). It's definitely invalid SQL grammar for a lot of good reasons.
Under the hood, I have no clue what SQlite does -- maybe it expands the * into fields and adds them to the GROUP BY under the hood? But its not a good SQL statement which is which it threw the error.
This returns the maxium value, not the complete record:
self.prices.maximum(:price_field)
And currently, I find the record like this:
def maximum_price
self.prices.find(:first, :conditions => "price = #{self.prices.maximum(:price_field)}" )
end
Is this the correct way ? Because the above needs two SQL statements to make it work, and it somehow does not feel right.
Ps. Additionally, I want that if more than one record has the same "maximum" value, then it should get the one with the latest updated_at value. So that would mean another SQL statement??
Pps. Does anyone know of a good or detailed reference for AREL and non-AREL things in Rails? The Rails Guide for ActiveRecord query is just not enough!
(I'm using Rails 3)
===UPDATE===
Using AREL I do the following:
self.prices.order("updated_at DESC").maximum(:price_field)
But this only gives the maximum value, not the complete record :(
Also, is the use of maximum() really AREL?
How about something like this?
self.prices.order("price DESC").first