I'm just beginning with ruby on rails and have a question regarding a bit more complex query. So far I've done simple queries while looking at rails guide and it worked really well.
Right now I'm trying to get some Ids from database and I would use those Ids to get the real objects and do something with them. Getting those is a bit more complex than simple Object.find method.
Here is how my query looks like :
select * from quotas q, requests r
where q.id=r.quota_id
and q.status=3
and r.text is not null
and q.id in
(
select A.id from (
select max(id) as id, name
from quotas
group by name) A
)
order by q.created_at desc
limit 1000;
This would give me 1000 ids when executing this query from sql manager. And I was thinking to obtain the list of ids first and then find objects by id.
Is there a way to get these objects directly by using this query? Avoiding ids lookup? I googled that you can execute query like this :
ActiveRecord::Base.connection.execute(query);
Assuming Quota has_many :requests,
Quota.includes(:requests).
where(status:3).
where('requests.text is not null').
where("quotas.id in (#{subquery_string_here})").
order('quotas.created_at desc').limit(1000)
I'm by no means an expert but most basic SQL functionality is baked into ActiveRecord. You might also want to look at the #group and #pluck methods for ways to eliminate the ugly string subquery.
Calling #to_sql on a relationship object will show you the SQL command it is equivalent to, and may help with your debugging.
I would use find_by_sql for this. I wouldn't swear that this is exactly right, but as I recall you can pretty much plonk an SQL statement into a find_by_sql and the resulting columns will be returned as attributes of an array of objects of the class you call it on:
status = 3
Quota.find_by_sql('
select *
from quotas q, requests r
where q.id=r.quota_id
and q.status= ?
and r.text is not null
and q.id in
(
select A.id from (
select max(id) as id, name
from quotas
group by name) A
)
order by q.created_at desc
limit 1000;', status)
If you come to Rails as someone used to writing raw SQL, you're probably better off using this syntax than stringing together a bunch of ActiveRecord methods - the result is the same, so it's just a matter of what you find more readable.
Btw, you shouldn't use string interpolation (i.e. #{variable} syntax) inside an SQL query. Use the '?' syntax instead (see my example) to avoid SQL injection potential.
Related
I'm trying to sort documents of type 'Case' by the 'Name' of the 'Contact' they belong to in Solr. But cases have no 'ContactName' field or similar, only 'ContactId'.
Only examples I could find are iterations of the example on this link: https://wiki.apache.org/solr/Join
But I couldn't apply it to my situation because of the sorting afterwards. The following gives me the cases I want but I can't sort it by the contact name afterwards because it only returns the fields of the cases.
{!join from=Id to=ContactId}*:*
SQL equivalent of what I want would be something like:
SELECT Case.Id, Contact.Name
FROM Case
LEFT JOIN Contact
ON Case.ContactId = Contact.Id
ORDER BY Contact.Name ASC;
So to answer my own question after some digging and a Solr training:
It is not best practice to use joins in a NoSql database like Solr. If you need joins, then your database is structured wrong. You should index everything you need, in the document itself, even if it is redundant. So in my case, I should index 'Contact.Name' field in my 'Case' documents.
Still, it is apparently possible to use SQL queries in Solr in case it is absolutely needed, if you're using SolrCloud but it doesn't support joins. However it is possible to work around that as follows:
SELECT s1.Id
FROM salesforce s1, salesforce s2
WHERE s1._type_ = 'Case' and s2._type_ = 'Contact' AND s1.ContactId = s2.Id
ORDER BY s2.Name ASC
It should be noted that the fields after '.' like the 'Id' in 's1.Id' must have 'docValues' activated in the schema. More info on docValues is here.
For the analytics of my site, I'm required to extract the 4 states of my users.
#members = list.members.where(enterprise_registration_id: registration.id)
# This pulls roughly 10,0000 records.. Which is evidently a huge data pull for Rails
# Member Load (155.5ms)
#invited = #members.where("user_id is null")
# Member Load (21.6ms)
#not_started = #members.where("enterprise_members.id not in (select enterprise_member_id from quizzes where quizzes.section_id IN (?)) AND enterprise_members.user_id in (select id from users)", #sections.map(&:id) )
# Member Load (82.9ms)
#in_progress = #members.joins(:quizzes).where('quizzes.section_id IN (?) and (quizzes.completed is null or quizzes.completed = ?)', #sections.map(&:id), false).group("enterprise_members.id HAVING count(quizzes.id) > 0")
# Member Load (28.5ms)
#completes = Quiz.where(enterprise_member_id: registration.members, section_id: #sections.map(&:id)).completed
# Quiz Load (138.9ms)
The operation returns a 503 meaning my app gives up on the request. Any ideas how I can refactor this code to run faster? Maybe by better joins syntax? I'm curious how sites with larger datasets accomplish what seems like such trivial DB calls.
The answer is your indexes. Check your rails logs (or check the console in development mode) and copy the queries to your db tool. Slap an "Explain" in front of the query and it will give you a breakdown. From here you can see what indexes you need to optimize the query.
For a quick pass, you should at least have these in your schema,
enterprise_members: needs an index on enterprise_member_id
members: user_id
quizes: section_id
As someone else posted definitely look into adding indexes if needed. Some of how to refactor depends on what exactly you are trying to do with all these records. For the #members query, what are you using the #members records for? Do you really need to retrieve all attributes for every member record? If you are not using every attribute, I suggest only getting the attributes that you actually use for something, .pluck usage could be warranted. 3rd and 4th queries, look fishy. I assume you've run the queries in a console? Again not sure what the queries are being used for but I'll toss in that it is often useful to write raw sql first and query on the db first. Then, you can apply your findings to rewriting activerecord queries.
What is the .completed tagged on the end? Is it supposed to be there? only thing I found close in the rails api is .completed? If it is a custom method definitely look into it. You potentially also have an use case for scopes.
THIRD QUERY:
I unfortunately don't know ruby on rails, but from a postgresql perspective, changing your "not in" to a left outer join should make it a little faster:
Your code:
enterprise_members.id not in (select enterprise_member_id from quizzes where quizzes.section_id IN (?)) AND enterprise_members.user_id in (select id from users)", #sections.map(&:id) )
Better version (in SQL):
select blah
from enterprise_members em
left outer join quizzes q on q.enterprise_member_id = em.id
join users u on u.id = q.enterprise_member_id
where quizzes.section_id in (?)
and q.enterprise_member_id is null
Based on my understanding this will allow postgres to sort both the enterprise_members table and the quizzes and do a hash join. This is better than when it will do now. Right now it finds everything in the quizzes subquery, brings it into memory, and then tries to match it to enterprise_members.
FIRST QUERY:
You could also create a partial index on user_id for your first query. This will be especially good if there are a relatively small number of user_ids that are null in a large table. Partial index creation:
CREATE INDEX user_id_null_ix ON enterprise_members (user_id)
WHERE (user_id is null);
Anytime you query enterprise_members with something that matches the index's where clause, the partial index can be used and quickly limit the rows returned. See http://www.postgresql.org/docs/9.4/static/indexes-partial.html for more info.
Thanks everyone for your ideas. I basically did what everyone said. I added indexes, resorted how I called everything, but the major difference was using the pluck method.. Here's my new stats :
#alt_members = list.members.pluck :id # 23ms
if list.course.sections.tests.present? && #sections = list.course.sections.tests
#quiz_member_ids = Quiz.where(section_id: #sections.map(&:id)).pluck(:enterprise_member_id) # 8.5ms
#invited = list.members.count('user_id is null') # 12.5ms
#not_started = ( #alt_members - ( #alt_members & #quiz_member_ids ).count #0ms
#in_progress = ( #alt_members & #quiz_member_ids ).count # 0ms
#completes = ( #alt_members & Quiz.where(section_id: #sections.map(&:id), completed: true).pluck(:enterprise_member_id) ).count # 9.7ms
#question_count = Quiz.where(section_id: #sections.map(&:id), completed: true).limit(5).map{|quiz|quiz.answers.count}.max # 3.5ms
I have a Custom Query that look like this
self.account.websites.find(:all,:joins => [:group_websites => {:group => :users}],:conditions=>["users.id =?",self])
where self is a User Object
I manage to generate the equivalent SQL for same
Here how it look
sql = "select * from websites INNER JOIN group_websites on group_websites.website_id = websites.id INNER JOIN groups on groups.id = group_websites.group_id INNER JOIN group_users ON (groups.id = group_users.group_id) INNER JOIN users on (users.id = group_users.user_id) where (websites.account_id = #{account_id} AND (users.id = #{user_id}))"
With the decent understanding of SQL and ActiveRecord I assumed that(which most would agree on) the result obtained from above query might take a longer time as compare to result obtained from find_by_sql(sql) one.
But Surprisingly
When I ran the above two
I found the ActiveRecord custom Query leading the way from ActiveRecord "find_by_sql" in term of load time
here are the test result
ActiveRecord Custom Query load time
Website Load (0.9ms)
Website Columns(1.0ms)
find_by_sql load time
Website Load (1.3ms)
Website Columns(1.0ms)
I repeated the test again an again and the result still the came out the same(with Custom Query winning the battle)
I know the difference aren't that big but still I just cant figure out why a normal find_by_sql query is slower than Custom Query
Can Anyone Share a light on this.
Thanks Anyway
Regards
Viren Negi
With the find case, the query is parameterized; this means the database can cache the query plan and will not need to parse and compile the query again.
With the find_by_sql case the entire query is passed to the database as a string. This means there is no caching that the database can do on the structure of the query, and it needs to be parsed and compiled on each occasion.
I think you can test this: try find_by_sql in this way (parameterized):
User.find_by_sql(["select * from websites INNER JOIN group_websites on group_websites.website_id = websites.id INNER JOIN groups on groups.id = group_websites.group_id INNER JOIN group_users ON (groups.id = group_users.group_id) INNER JOIN users on (users.id = group_users.user_id) where (websites.account_id = ? AND (users.id = ?))", account_id, users.id])
Well, the reason is probably quite simple - with custom SQL, the SQL query is sent immediately to db server for execution.
Remember that Ruby is an interpreted language, therefore Rails generates a new SQL query based on the ORM meta language you have used before it can be sent to the actual db server for execution. I would say additional 0.1 ms is the time taken by framework to generate the query.
Can anyone explain this?
Project.includes([:user, :company])
This executes 3 queries, one to fetch projects, one to fetch users for those projects and one to fetch companies.
Project.select("name").includes([:user, :company])
This executes 3 queries, and completely ignores the select bit.
Project.select("user.name").includes([:user, :company])
This executes 1 query with proper left joins. And still completely ignores the select.
It would seem to me that rails ignores select with includes. Ok fine, but why when I put a related model in select does it switch from issuing 3 queries to issuing 1 query?
Note that the 1 query is what I want, I just can't imagine this is the right way to get it nor why it works, but I'm not sure how else to get the results in one query (.joins seems to only use INNER JOIN which I do not in fact want, and when I manually specifcy the join conditions to .joins the search gem we're using freaks out as it tries to re-add joins with the same name).
I had the same problem with select and includes.
For eager loading of associated models I used native Rails scope 'preload' http://apidock.com/rails/ActiveRecord/QueryMethods/preload
It provides eager load without skipping of 'select' at scopes chain.
I found it here https://github.com/rails/rails/pull/2303#issuecomment-3889821
Hope this tip will be helpful for someone as it was helpful for me.
Allright so here's what I came up with...
.joins("LEFT JOIN companies companies2 ON companies2.id = projects.company_id LEFT JOIN project_types project_types2 ON project_types2.id = projects.project_type_id LEFT JOIN users users2 ON users2.id = projects.user_id") \
.select("six, fields, I, want")
Works, pain in the butt but it gets me just the data I need in one query. The only lousy part is I have to give everything a model2 alias since we're using meta_search, which seems to not be able to figure out that a table is already joined when you specify your own join conditions.
Rails has always ignored the select argument(s) when using include or includes. If you want to use your select argument then use joins instead.
You might be having a problem with the query gem you're talking about but you can also include sql fragments using the joins method.
Project.select("name").joins(['some sql fragement for users', 'left join companies c on c.id = projects.company_id'])
I don't know your schema so i'd have to guess at the exact relationships but this should get you started.
I might be totally missing something here but select and include are not a part of ActiveRecord. The usual way to do what you're trying to do is like this:
Project.find(:all, :select => "users.name", :include => [:user, :company], :joins => "LEFT JOIN users on projects.user_id = users.id")
Take a look at the api documentation for more examples. Occasionally I've had to go manual and use find_by_sql:
Project.find_by_sql("select users.name from projects left join users on projects.user_id = users.id")
Hopefully this will point you in the right direction.
I wanted that functionality myself,so please use it.
Include this method in your class
#ACCEPTS args in string format "ASSOCIATION_NAME:COLUMN_NAME-COLUMN_NAME"
def self.includes_with_select(*m)
association_arr = []
m.each do |part|
parts = part.split(':')
association = parts[0].to_sym
select_columns = parts[1].split('-')
association_macro = (self.reflect_on_association(association).macro)
association_arr << association.to_sym
class_name = self.reflect_on_association(association).class_name
self.send(association_macro, association, -> {select *select_columns}, class_name: "#{class_name.to_sym}")
end
self.includes(*association_arr)
end
And you will be able to call like: Contract.includes_with_select('user:id-name-status', 'confirmation:confirmed-id'), and it will select those specified columns.
The preload solution doesn't seem to do the same JOINs as eager_load and includes, so to get the best of all worlds I also wrote my own, and released it as a part of a data-related gem I maintain, The Brick.
By overriding ActiveRecord::Associations::JoinDependency.apply_column_aliases() like this then when you add a .select(...) then it can act as a filter to choose which column aliases get built out.
With gem 'brick' loaded, in order to enable this selective behaviour, add the special column name :_brick_eager_load as the first entry in your .select(...), which turns on the filtering of columns while the aliases are being built out. Here's an example:
Employee.includes(orders: :order_details)
.references(orders: :order_details)
.select(:_brick_eager_load,
'employees.first_name', 'orders.order_date', 'order_details.product_id')
Because foreign keys are essential to have everything be properly associated, they are automatically added, so you do not need to include them in your select list.
Hope it can save you both query time and some RAM!
In rails I have 2 tables:
bans(ban_id, admin_id)
ban_reasons(ban_reason_id, ban_id, reason_id)
I want to find all the bans for a certain admin where there is no record in the ban_reasons table. How can I do this in Rails without looping through all the ban records and filtering out all the ones with ban.ban_reasons.nil? I want to do this (hopefully) using a single SQL statement.
I just need to do: (But I want to do it the "rails" way)
SELECT bans.* FROM bans WHERE admin_id=1234 AND
ban_id NOT IN (SELECT ban_id FROM ban_reasons)
Your solution works great (only one request) but it's almost plain SQL:
bans = Ban.where("bans.id NOT IN (SELECT ban_id from ban_reason)")
You may also try the following, and let rails do part of the job:
bans = Ban.where("bans.id NOT IN (?)", BanReason.select(:ban_id).map(&:ban_id).uniq)
ActiveRecord only gets you to a point, everything after should be done by raw SQL. The good thing about AR is that it makes it pretty easy to do that kind of stuff.
However, since Rails 3, you can do almost everything with the AREL API, although raw SQL may or may not look more readable.
I'd go with raw SQL and here is another query you could try if yours doesn't perform well:
SELECT b.*
FROM bans b
LEFT JOIN ban_reason br on b.ban_id = br.ban_id
WHERE br.ban_reason_id IS NULL
Using Where Exists gem (which I'm author of):
Ban.where(admin_id: 123).where_not_exists(:ban_reasons)