I'm using rails and am trying to figure out how to use ActiveRecord within the method to combine the following into one query:
def children_active(segment)
parent_id = Category.select('id').where('segment' => segment)
Category.where('parent_id'=>parent_id, 'active' => true)
end
Basically, I'm trying to get sub categories of a category that is designated by a unique column called segment. Right now, I'm getting the id of the category in the first query, and then using that value for the parent_id in the second query. I've been trying to figure out how to use AR to do a join so that it can be accomplished in just one query.
You can use self join with a alias table name:
Category.joins("LEFT OUTER JOIN categories AS segment_categories on segment_categories.id = categories.parent_id").where("segment_categories.segment = ?", segment).where("categories.active = ?", true)
This may looks not so cool, but it can implement the query in one line, and there will be much less performance loss than your solution when data collection is big, because "INCLUDE IN" is much more slower than "JOIN".
Related
I have a Company model that has_many Statement.
class Company < ActiveRecord::Base
has_many :statements
end
I want to get statements that have most latest date field grouped by fiscal_year_end field.
I implemented the function like this:
c = Company.first
c.statements.to_a.group_by{|s| s.fiscal_year_end }.map{|k,v| v.max_by(&:date) }
It works ok, but if possible I want to use ActiveRecord query(SQL), so that I don't need to load unnecessary instance to memory.
How can I write it by using SQL?
select t.username, t.date, t.value
from MyTable t
inner join (
select username, max(date) as MaxDate
from MyTable
group by username
) tm on t.username = tm.username and t.date = tm.MaxDate
For these kinds of things, I find it helpful to get the raw SQL working first, and then translate it into ActiveRecord afterwards. It sounds like a textbook case of GROUP BY:
SELECT fiscal_year_end, MAX(date) AS max_date
FROM statements
WHERE company_id = 1
GROUP BY fiscal_year_end
Now you can express that in ActiveRecord like so:
c = Company.first
c.statements.
group(:fiscal_year_end).
order(nil). # might not be necessary, depending on your association and Rails version
select("fiscal_year_end, MAX(date) AS max_date")
The reason for order(nil) is to prevent ActiveRecord from adding ORDER BY id to the query. Rails 4+ does this automatically. Since you aren't grouping by id, it will cause the error you're seeing. You could also order(:fiscal_year_end) if that is what you want.
That will give you a bunch of Statement objects. They will be read-only, and every attribute will be nil except for fiscal_year_end and the magically-present new field max_date. These instances don't represent specific statements, but statement "groups" from your query. So you can do something like this:
- #statements_by_fiscal_year_end.each do |s|
%tr
%td= s.fiscal_year_end
%td= s.max_date
Note there is no n+1 query problem here, because you fetched everything you need in one query.
If you decide that you need more than just the max date, e.g. you want the whole statement with the latest date, then you should look at your options for the greatest n per group problem. For raw SQL I like LATERAL JOIN, but the easiest approach to use with ActiveRecord is DISTINCT ON.
Oh one more tip: For debugging weird errors, I find it helpful to confirm what SQL ActiveRecord is trying to use. You can use to_sql to get that:
c = Company.first
puts c.statements.
group(:fiscal_year_end).
select("fiscal_year_end, MAX(date) AS max_date").
to_sql
In that example, I'm leaving off order(nil) so you can see that ActiveRecord is adding an ORDER BY clause you don't want.
for example you want to get all statements by start of the months you should use this
#companey = Company.first
#statements = #companey.statements.find(:all, :order => 'due_at, id', :limit => 50)
then group them as you want
#monthly_statements = #statements.group_by { |statement| t.due_at.beginning_of_month }
Building upon Bharat's answer you can do this type of query in Rails using find_by_sql in this way:
Statement.find_by_sql ["Select t.* from statements t INNER JOIN (
SELECT fiscal_year_end, max(date) as MaxDate GROUP BY fiscal_year_end
) tm on t.fiscal_year_end = tm.fiscal_year_end AND
t.created_at = tm.MaxDate WHERE t.company_id = ?", company.id]
Note the last where part to make sure the statements belong to a specific company instance, and that this is called from the class. I haven't tested this with the array form, but I believe you can turn this into a scope and use it like this:
# In Statement model
scope :latest_from_fiscal_year, lambda |enterprise_id| {
find_by_sql[..., enterprise_id] # Query above
}
# Wherever you need these statements for a particular company
company = Company.find(params[:id])
latest_statements = Statement.latest_from_fiscal_year(company.id)
Note that if you somehow need all the latest statements for all companies then this most likely leave you with a N+1 queries problem. But that is a beast for another day.
Note: If anyone else has a way to have this query work on the association without using the last where part (company.statements.latest_from_year and such) let me know and I'll edit this, in my case in rails 3 it just pulled em from the whole table without filtering.
My title might be confusing, I wasn't sure what to write.
In rails I understand how to fetch Many Objects for One parent object
#first_user = User.first
#first_user_posts = #first_user.posts
But how can I fetch Many Objects for Many parent objects and select its attributes in one query?. I am trying to do something like that:
#many_posts = Post.all
#posts_by_user_gender = #many_posts.joins(:user).map(&:gender)
hoping it would give me an array that could look something like this:
#posts_by_user_gender => ["male", nil, "female", nil]
#I know I can do this map technique if I fetch it directly from the User model
# User.all.map(&:gender),
# but I want to start with those that posted in a specific category
# Post.where(:category_id => 1)
and then to count the males I could use the Ruby Array method .count
#males_count_who_posted = #posts_by_user_gender.count("male")
=> 1
I could always do 3 separate queries
#males_count_who_posted = #many_posts.select(:user_id).joins(:user)
.where("gender = ?", "male").count
#females_count_who_posted = ...
but I find that extremely inefficient, especially if I do the same for something like "industry" where you could have more than 3 options.
you can join model via SQL syntax
#posts_by_user_gender = #many_posts.joins("LEFT JOIN Users where users.id=posts.user_id").joins("LEFT JOIN Genders where genders.id=user.gender_id")
In my app I'm obtaining a certain category, and I'm filtering the associated items based on their name.
The following code should be pretty clear:
categories = Category.where(:id => params[:category_id]).includes(:items).where("lower(items.name) like ?", "%#{params[:keywords].downcase}%")
However, if the name filter excludes all the items, the categories object returned by where is nil. Is this the expected behaviour? How can I get the category even either items exist or not?
The easiest way might be to just split the query:
#category = Category.find(params[:category_id])
#items = #category.items.where("lower(items.name) like ?", "%#{params[:keywords].downcase}%")
Based on your code it seems like category_id references only 1 category so I've changed it to singular.
You should look into doing an OUTER JOIN against the items table which will give you categories regardless of whether or not their items meet the name filter.
I have a model Category, which has_many Products, and a Product in turn has_many Categories. When a user searches for a Category, I'd like to return the products of the matching Categories without losing my Arel object. Here's what I have so far:
Category.where("upper(title) like ?", search_term.upcase).map {|category| category.products}.flatten
This does the trick of returning the products, but of course what I have is an array and not Arel. I can get as far as adding an :includes(:products) clause, so I do indeed get the products back but I still have them attached to their categories. How do I adjust my query so that all I get back is an Arel that only addresses products?
If it is products that you want then you should probably start with the Product object when you are searching. For example ,you could do it like this:
Product.joins(:categories).where("upper(categories.title) like ?", search_term.upcase)
The reason I use joins instead of includes is that joins will perform an INNER JOIN instead of LEFT OUTER JOIN which is what you need to only return the products that are actually associated with the found categories.
To make it a little more elegant you could wrap it all up in a scope in your Product model like this:
# In Product.rb
scope :in_categories_like, Proc.new{ |search_term|
joins(:categories).where("upper(categories.title) like ?", search_term.upcase)
}
# In use
#products = Product.in_categories_like(params[:search_term])
I'm trying to create basic search in my web app. Here's code of search function.
def self.search(title, category_id, city_id)
if title || category_id || city_id
joins(:category).where('title LIKE (?) AND category.category_id IN (?) AND city.city_id IN (?)', "%#{title}%", "%#{category_id}%", "%#{city_id}%")
else
scoped
end
end
I have these associations in my model:
has_one :category
has_one :city
And I get this error
ActionView::Template::Error (PG::Error: ERROR: missing FROM-clause entry for ta
ble "category"
LINE 1: ..._id" = "events"."id" WHERE (title LIKE ('%%') AND category.c...
I'm using PostgreSQL. What I can do to remove this error?
The form of joins that you're using wants the association name, the SQL wants the table name. The table should be called categories.
A few other things:
I don't see you joining :city anywhere so your next error will be "Missing FROM-clause entry for table "city". The solution will be to .joins(:city) and use cities in the where. But keep reading anyway.
You don't need the parentheses around the value for LIKE, just title LIKE ? is fine.
You're using IN expressions for the category and city but you're giving them LIKE patterns and that won't work: the IDs will be numbers and you can use LIKE with numbers. If you're using IN then you'll usually want to supply a list of possible values, if you only want to match one value then just use = and a single value for the placeholder.
The categories table probably doesn't have a category_id column, similarly for the cities table and city_id column. Those two columns should be in your model's table.
Searching for a title when you don't have a title doesn't make much sense. Similarly for country and city.
That looks like a lot of problems but they can be fixed without too much effort:
def self.search(title, category_id, city_id)
rel = scoped
rel = rel.where('title like ?', "%#{title}%") if(title)
rel = rel.where('category_id = ?', category_id) if(category_id)
rel = rel.where('city_id = ?', city_id) if(city_id)
rel
end
and you don't even need joins or explicit table names at all.