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.
Related
I want to display the SQL used in a count. However, Model.count.to_sql will not work because count returns a FixNum that doesn't have a to_sql method. I think the simplest solution is to do this:
Model.where(nil).to_sql.sub(/SELECT.*FROM/, "SELECT COUNT(*) FROM")
This creates the same SQL as is used in Model.count, but is it going to cause a problem further down the line? For example, if I add a complicated where clause and some joins.
Is there a better way of doing this?
You can try
Model.select("count(*) as model_count").to_sql
You may want to dip into Arel:
Model.select(Arel.star.count).to_sql
ASIDE:
I find I often want to find sub counts, so I embed the count(*) into another query:
child_counts = ChildModel.select(Arel.star.count)
.where(Model.arel_attribute(:id).eq(
ChildModel.arel_attribute(:model_id)))
Model.select(Arel.star).select(child_counts.as("child_count"))
.order(:id).limit(10).to_sql
which then gives you all the child counts for each of the models:
SELECT *,
(
SELECT COUNT(*)
FROM "child_models"
WHERE "models"."id" = "child_models"."model_id"
) child_count
FROM "models"
ORDER BY "models"."id" ASC
LIMIT 10
Best of luck
UPDATE:
Not sure if you are trying to solve this in a generic way or not. Also not sure what kind of scopes you are using on your Model.
We do have a method that automatically calls a count for a query that is put into the ui layer. I found using count(:all) is more stable than the simple count, but sounds like that does not overlap your use case. Maybe you can improve your solution using the except clause that we use:
scope.except(:select, :includes, :references, :offset, :limit, :order)
.count(:all)
The where clause and the joins necessary for the where clause work just fine for us. We tend to want to keep the joins and where clause since that needs to be part of the count. While you definitely want to remove the includes (which should be removed by rails automatically in my opinion), but the references (much trickier especially in the case where it references a has_many and requires a distinct) that starts to throw a wrench in there. If you need to use references, you may be able to convert these over to a left_join.
You may want to double check the parameters that these "join" methods take. Some of them take table names and others take relation names. Later rails version have gotten better and take relation names - be sure you are looking at the docs for the right version of rails.
Also, in our case, we spend more time trying to get sub selects with more complicated relationships, we have to do some munging. Looks like we are not dealing with where clauses as much.
ref2
I'm setting up an AR query that is basically meant to find an average of a few values that span three different tables. I'm getting hung up on how to embed the result of a particular Count query inside of the Active Record select statement.
Just by itself, this query returns "3":
Order.where(user_id: 319).count => 3
My question is, can I embed this into a select statement as a SQL alias similar to below:
Table.xxxxxx.select("Order.where(user_id: 319).count AS count,user_id, SUM(quantity*current_price) AS revenue").xxxxx
It seems to be throwing an error and generally not recognizing what I'm trying to do when I declare that first count alias. Any ideas on the syntax?
Well, after examining a bit, I cleared my mind into the ActiveRecord select() syntax.
It's a method that can take a variable length of parameters. So, your failing :
Table.xxxxxx.select("Order.where(user_id: 319).count AS count,user_id, SUM(quantity*current_price) AS revenue").xxxxx
After replacing proper SQL for your misplaced ActiveRecord statement, should be more of like this [be careful, you can't use as count in most cases, count is reserved]:
Table.xxxxx.select("(SELECT count(id) from orders where user_id=319) as usercount", "user_id","SUM(quantity*current_price) AS revenue").xxxx
But I guess you should need more a per-user_id-table.
So, I'd skip Models and go to direct SQL, always being careful to avoid injections:
ActiveRecord::Base.connection.execute('SELECT COUNT(orders.id) as usercount, users.id from users, orders where users.id=orders.user_id group by users.id')
This is simplified of course, you can apply the rest of the data (which I currently do not know) accordingly. The above simplified, not full solution, could be written also as:
Order.joins(:user).select("count(orders.id) as usercount, users.id").group(:user_id)
Helo,
My query:
#county = County.joins(:state)
.where("counties.slug = ? AND states.slug = ?", params[:county_slug])
.select('states.*, counties.*')
.first!
From the log, the SQL looks like this:
SELECT states.*, counties.* FROM "counties" INNER JOIN "states" ON "states"."id" = "counties"."state_id" LIMIT 1
My problem is that is doesn't eager load the data from the associated table (states), because when I do, for example, #county.state.name, it runs another query, although, as you can see from the log, it had already queried the database for the data in that table as well. But it doesn't pre populate #county.state
Any idea how i can get all the data from the database in just ONE query?
Thx
I think you need to use include instead of joins to get the eager loading. There's a good railscasts episode about the differences: http://railscasts.com/episodes/181-include-vs-joins , in particular:
The question we need to ask is “are we using any of the related model’s attributes?” In our case the answer is “yes” as we’re showing the user’s name against each comment. This means that we want to get the users at the same time as we retrieve the comments and so we should be using include here.
I have an app that has a number of Post models, each of which belongs_to a User model. When these posts are published, a PublishedPost model is created that belongs_to the relevant Post model.
I'm trying to build an ActiveRecord query to find published posts that match a user name, then get the ids of those published posts, but I'm getting an error when I try to use the pluck method after eager-loading my associations and searching them with the where method.
Here's (part of) my controller:
class PublishedPostsController < ApplicationController
def index
ar_query = PublishedPost.order("published_posts.created_at DESC")
if params[:searchQuery].present?
search_query = params[:searchQuery]
ar_query = ar_query.includes(:post => :user)
.where("users.name like ?", "%#{search_query}%")
end
#found_ids = ar_query.pluck(:id)
...
end
end
When the pluck method is called, I get this:
ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'users.name' in 'where clause': SELECT id FROM `published_posts` WHERE (users.name like '%Andrew%') ORDER BY published_posts.created_at DESC
I can get the results I'm looking for with
#found_ids = ar_query.select(:id).map{|r| r.id}
but I'd rather use pluck as it seems like the cleaner way to go. I can't figure out why it's not working, though. Any ideas?
You need to and should do joins instead of includes here.
The two functions are pretty similar except that the data from joins is not returned in the result of the query whereas the data in an includes is.
In that respect, includes and pluck are kind of antithetical. One says to return me all the data you possibly can, whereas the other says to only give me only this one little bit.
Since you only want a small amount of the data, you want to do joins. (Strangely select which also seems somewhat antithetical still works, but you would need to remove the ambiguity over id in this case.)
Try it out in the console and you'll see that includes causes a query that looks kind of like this: SELECT "posts"."id" as t0_ro, "posts"."text" as t0_r1, "users"."id" as t1_r0, "users"."name" as t1_r1 ... When you tack on a pluck statement all those crazy tx_ry columns go away and are replaced by whatever you specified.
I hope that helps, but if not maybe this RailsCast can. It is explained around the 5 minute mark.
http://railscasts.com/episodes/181-include-vs-joins
If you got here by searching "rails pluck ambiguous column", you may want to know you can just replace query.pluck(:id) with:
query.pluck("table_name.id")
Your query wouldn't work as it is written, even without the pluck call.
Reason being, your WHERE clause includes literal SQL referencing the users table which Rails doesn't notice and decides to use multiple queries and join in memory ( .preload() ) instead of joining in the database level ( .eager_load() ):
SELECT * from published_posts WHERE users.name like "pattern" ORDER BY published_posts.created_at DESC
SELECT * from posts WHERE id IN ( a_list_of_all_post_ids_in_publised_posts_returned_above )
SELECT * from users WHERE id IN ( a_list_of_all_user_ids_in_posts_returned_above )
The first of the 3 queries fails and it is the error you get.
To force Rails use a JOIN here, you should either use the explicit .eager_load() instead of .includes(), or add a .references() clause.
Other than that, what #Geoff answered stands, you don't really need to .includes() here, but rather a .joins().
Say I have a Post model. When I delete last post 'Post 24', I want the next post to take id of Post 24 and not Post 25.
I want to show id in views and I don't want missing numbers. How do I do that?
Thanks for your help.
The purpose of an id is to be nothing more than an internal identifier. It shouldn't be used publicly at all. This isn't a Rails thing, but a database issue. MySQL won't reclaim id's because it can lead to very serious complications in your app. If a record is deleted, its id is laid to rest forevermore, so that no future record will be mistaken for it.
However, there is a way to do what you want. I believe you want a position integer column instead. Add that to your model/table, and then install the acts_as_list plugin.
Install it the usual way:
script/plugin install git://github.com/rails/acts_as_list.git
Then add the "hook" to your model:
class Post < ActiveRecord::Base
acts_as_list
end
Now the position column of your post model will automatically track itself, with no sequence gaps. It'll even give you some handy methods for re-ordering if you so choose.
Conversely, you could let the SQL do this itself:
SELECT rownum AS id, [whatever other columns you want]
FROM posts_table
WHERE [conditions]
ORDER BY [ordering conditions]
This will add numbers to each row without skipping any like you said.
NOTE: I use Oracle. I don't know if this exact code will work in other flavors.