I am using the kaminari gem for pagination. I have a resources controller which paginates perfectly (due to the simple nature of the ordering). That can be seen here:
#resources = Resource.order("created_at desc").page(params[:page]).per(25)
That just sorts them by latest first. when i do .class it appears thats an activerecord::relation
On my tags though, I want to sort them by a relationship (the number of resources assigned to that tag)
#tags = Tag.all.sort{|a, b| b.number_of_resources <=> a.number_of_resources}.page(params[:page]).per(50)
It gives me the error however undefined methodpage' for #`
Tag.all returns an Array, hence your #page call failing, as it expects an ARel relation.
If #number_of_resources maps to a DB column, then all you need to do is:
Tag.order('number_of_resources').page(params[:page]).per(50)
If it's not, you either need to add it to the Tag database table, or just do your sort/paginate in Ruby rather than using kaminari. This will be feasible if the number of tags is under ~1000 or so.
If you do add the info to the db, check out this post: Counter Cache for a column with conditions?
you should do something like: 1) joins the two tables, 2) group rows by tag, 3) count how many rows belongs to each group, 4) order using that new column with the count
you should make a good sql statement and then you can call pagination
Related
I would like to order a collection first by priority and then due time like this:
#ods = Od.order(:priority, :due_date_time)
The problem is due_date_time is an instance method of Od, so I get
PG::UndefinedColumn: ERROR: column ods.due_date_time does not exist
I have tried the following, but it seems that by sorting and mapping ids, then finding them again with .where means the sort order is lost.
#ods = Od.where(id: (Od.all.sort {|a,b| a.due_date_time <=> b.due_date_time}.map(&:id))).order(:priority)
due_date_time calls a method from a child association:
def due_date_time
run.cut_off_time
end
run.cut_off_time is defined here:
def cut_off_time
(leave_date.beginning_of_day + route.cut_off_time_mins_since_midnight * 60)
end
I'm sure there is an easier way. Any help much appreciated! Thanks.
order from ActiveRecord similar to sort from ruby. So, Od.all.sort run iteration after the database query Od.all, run a new iteration map and then send a new database query. Also Od.all.sort has no sense because where select record when id included in ids but not searching a record for each id.
Easier do something like this:
Od.all.sort_by { |od| [od.priority, od.due_date_time] }
But that is a slow solution(ods table include 10k+ records). Prefer to save column to sort to the database. When that is not possible set logic to calculate due_date_time in a database query.
I am hardly trying to find one comparison of result.blank? and result[0] so finally today when i was checking one query with these two methods.
Here the code, result variable is #categories, which is an ActiveRecord result
This blank check calling one extra db call like SELECT COUNT(*) AS count_all
if #categories.blank?
end
But here that extra query is not showing there.
if #categories[0]
end
Is there any logic behind that? I couldn't find that
It is important to note that assigning a ActiveRecord query to a variable does not return the result of the query. Something like this:
#categories = Category.where(public: true)
Does not return an array with all categories that are public. Instead it returns an Relation which defines an query. The query to the database is execute once you call a method in the relation that needs to return the actual record, for example each, load, count.
That said: When you call blank? on a relation Rails needs to know it the relation will not return an empty array. Therefore Rails executes an query like:
SELECT COUNT(*) FROM categories WHERE public = 1
Because that queries is much faster that fetching all records when the only thing you need to know if there are any matching records.
Whereas #categories[0] works differently. Here it need to load all records to have an array holding all macthing categories and than return the first record in that array.
At this point both version ran only on query to the database. But I guess your next step would be to iterate over the records if there were any. If you used the first version (blank?) then the objects were not loaded, they were only counted. Therefore Rails would need to query for the actual records, what would result in a second query. The second exmaple ([0]) has the records already loaded, therefore not seconds query in needed.
I have a model Company that have columns pbr, market_cap and category.
To get averages of pbr grouped by category, I can use group method.
Company.group(:category).average(:pbr)
But there is no method for weighted average.
To get weighted averages I need to run this SQL code.
select case when sum(market_cap) = 0 then 0 else sum(pbr * market_cap) / sum(market_cap) end as weighted_average_pbr, category AS category FROM "companies" GROUP BY "companies"."category";
In psql this query works fine. But I don't know how to use from Rails.
sql = %q(select case when sum(market_cap) = 0 then 0 else sum(pbr * market_cap) / sum(market_cap) end as weighted_average_pbr, category AS category FROM "companies" GROUP BY "companies"."category";)
ActiveRecord::Base.connection.select_all(sql)
returns a error:
output error: #<NoMethodError: undefined method `keys' for #<Array:0x007ff441efa618>>
It would be best if I can extend Rails method so that I can use
Company.group(:category).weighted_average(:pbr)
But I heard that extending rails query is a bit tweaky, now I just want to know how to run the result of sql from Rails.
Does anyone knows how to do it?
Version
rails: 4.2.1
What version of Rails are you using? I don't get that error with Rails 4.2. In Rails 3.2 select_all used to return an Array, and in 4.2 it returns an ActiveRecord::Result. But in either case, it is correct that there is no keys method. Instead you need to call keys on each element of the Array or Result. It sounds like the problem isn't from running the query, but from what you're doing afterward.
In any case, to get the more fluent approach you've described, you could do this:
class Company
scope :weighted_average, lambda{|col|
select("companies.category").
select(<<-EOQ)
(CASE WHEN SUM(market_cap) = 0 THEN 0
ELSE SUM(#{col} * market_cap) / SUM(market_cap)
END) AS weighted_average_#{col}
EOQ
}
This will let you say Company.group(:category).weighted_average(:pbr), and you will get a collection of Company instances. Each one will have an extra weighted_average_pbr attribute, so you can do this:
Company.group(:category).weighted_average(:pbr).each do |c|
puts c.weighted_average_pbr
end
These instances will not have their normal attributes, but they will have category. That is because they do not represent individual Companies, but groups of companies with the same category. If you want to group by something else, you could parameterize the lambda to take the grouping column. In that case you might as well move the group call into the lambda too.
Now be warned that the parameter to weighted_average goes straight into your SQL query without escaping, since it is a column name. So make sure you don't pass user input to that method, or you'll have a SQL injection vulnerability. In fact I would probably put a guard inside the lambda, something like raise "NOPE" unless col =~ %r{\A[a-zA-Z0-9_]+\Z}.
The more general lesson is that you can use select to include extra SQL expressions, and have Rails magically treat those as attributes on the instances returned from the query.
Also note that unlike with select_all where you get a bunch of hashes, with this approach you get a bunch of Company instances. So again there is no keys method! :-)
I have a count column in my tags table. I wanna increment tag count if a tag is just added to the post while updating, and it's already inside db. I added this to my post model:
before_update :increment_tag
def increment_tag
db_post = Post.find_by_id(self)
self.tags.each do |tag|
unless db_post.tags.include? tag
tag.update_attribute("count", tag.count + 1)
end
end
end
I get the post from db and test if the current tag is already in db, if it is, nothing happens, if it's not there, it should update count field. But for some reason this doesn't work.
You should not have a Count column in a tag. You should in stead set up your models propperly so you could do the following:
db_post.tags.count
If you do it right, you can get this in your tag:
tag.post.tags.count
If your aim is to find how many times the tag is used in a post, in total, you can simply count the instances in the TagToPostColumn (if you got one), which you need to sine this is a many-to-many relation.
Then you do:
TagToPostColumn.where(tag_id: someTag.id).count
count is a standard attribute, and you should never have to keep track of this yourself, unless you actually need a column called count that tracks something other than the models you have in the database. But then it is a good idea to name it something else than count, since it can lead to ambiguous attributes.
Also, i find it very strange that you are doing this:
db_post = Post.find_by_id(self)
Why are you not just using the self parameter, in stead of doing a db lookup to find the post you already have.
So I am pulling my hair over this issue / gotcha. Basically I used find_by_sql to fetch data from my database. I did this because the query has lots of columns and table joins and I think using ActiveRecord and associations will slow it down.
I managed to pull the data and now I wanted to modify returned values. I did this by looping through the result ,for example.
a = Project.find_by_sql("SELECT mycolumn, mycolumn2 FROM my_table").each do |project|
project['mycolumn'] = project['mycolumn'].split('_').first
end
What I found out is that project['mycolumn'] was not changed at all.
So my question:
Does find_by_sql return an array Hashes?
Is it possible to modify the value of one of the attributes of hash as stated above?
Here is the code : http://pastie.org/4213454 . If you can have a look at summarize_roles2() that's where the action is taking place.
Thank you. Im using Rails 2.1.1 and Ruby 1.8. I can't really upgrade because of legacy codes.
Just change the method above to access the values, print value of project and you can clearly check the object property.
The results will be returned as an array with columns requested encapsulated as attributes of the model you call this method from.If you call Product.find_by_sql then the results will be returned in a Product object with the attributes you specified in the SQL query.
If you call a complicated SQL query which spans multiple tables the columns specified by the SELECT will be attributes of the model, whether or not they are columns of the corresponding table.
Post.find_by_sql "SELECT p.title, c.author FROM posts p, comments c WHERE p.id = c.post_id"
> [#<Post:0x36bff9c #attributes={"title"=>"Ruby Meetup", "first_name"=>"Quentin"}>, ...]
Source: http://api.rubyonrails.org/v2.3.8/
Have you tried
a = Project.find_by_sql("SELECT mycolumn, mycolumn2 FROM my_table").each do |project|
project['mycolumn'] = project['mycolumn'].split('_').first
project.save
end