We have an SQL query in our Rails 3 app.
#followers returns an array of IDs of users following the current_user.
#followers = current_user.following
#feed_items = Micropost.where("belongs_to_id IN (?)", #followers)
Is there a more efficient way to do this query?
The query you have can't really be optimized anymore than it is. It could be made faster by adding an index to belongs_to_id (which you should almost always do for foreign keys anyway), but that doesn't change the actual query.
There is a cleaner way to write IN queries though:
Micropost.where(:belongs_to_id => #followers)
where #followers is an array of values for belongs_to_id.
It looks good to me.
However if you're looking for real minimum numer of characters in the code, you could change:
Micropost.where("belongs_to_id IN (?)", #followers)
to
Micropost.where("belongs_to_id = ?", #followers)
which reads a little easier.
Rails will see the array and do the IN.
As always the main goal of the ruby language is readability so little improvements help.
As for query being inefficent, you shuld check into indexs on that field.
They tend to be a little more specific for each db - you have only specified generic sql. in your question.
Related
Suppose I have Posts and posts' Comments. I want to filter all the Posts that have more than 10 comments. I began writing something like Posts.includes(:comments).group("post.id").count("comments.id"), to obtain a hash of posts and their counts, and I can extract the information from there, but I want some one-line straightforward way to do that
Sure I can use some pure sql syntax statements, but I want it in a pure rails way. Any idea ?
Assuming the models are named in the more typical singular form of Post and Comment and have the usual association relationship, then the following should work:
Post.joins(:comments).group('posts.id').having('count(comments.id) > 10')
I ask because a thorough Google search returns no clue as to how to do this.
I am trying to pull an example of a column field which is rarely used and is unfortunately littered with newlines and dashes even in empty ones, so I can't just ask for ones that have data. I need to ask for a column that has at least 10-15 characters or something like this. I can also imagine this query being useful for validating pre-existing data. I know about the validator that does this, but I'm not trying to validate, I'm trying to search.
Thanks!
Seems ActiveRecord does not support this. But you can do it anyway like (Mysql example)
Model.where("CHAR_LENGTH(text_field) = ?", 10)
in Postgres the same should work but in documentation it says to use char_length()
Also what you could do is on saving the record store the size of the field with a callback
before_save {|r| r.text_field_size = r.text_field.size}
With this you can now query with that, wich will be DB agnostic.
Model.where(text_field_size: 10)
I think you'll have to write so part of the request in SQL.
For MySQL, use something like :
Model.where("CHAR_LENGTH(field_name) >= ?", min_length)
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 got some problems here, I can't make my find_by_sql request to render an ActiveRecord relation. Indeed, I need an activerecord relation to make a new request:
#searches = #searches.find_by_sql('SELECT *, COUNT( follower_id ) FROM follows GROUP BY followable_id LIMIT 0 , 3') if params[:only_famous_projects]
#project_pages = #project_pages.where(:project_id => #searches.pluck(:'followable.id')) if params[:only_famous_projects]
I can't use "pluck" without an activerecord relation. Therefore, I think I have to convert my sql request to an Activerecord request. However, as soon as I use "count" on ActiveRecord, I have an huge problem: I don't have on the end an ActiveRecord relation, but a FixNum!
I don't know where to find the answer anymore, I would be really gratefull if you could help me.
Thanks
find_by_sql will return an ActiveRecord object only if you call it with YourModel.find_by_sql.
Why not use the ActiveRecord query interface. It does a good job with calculations.
UPDATED
#searches = #searches.group(:followable_id).limit(3).offset(0).count(:follower_id) if params[:only_famous_projects]
Notice that it will give you a Hash containing the count for each followable_id.
Isn't LIMIT 0, 3 equivalent to LIMIT 3 ?
COUNT will always return a FixNUM, because you asked the database to count the number of rows.
You should really try to use find_by_sql as a last resort as it is only meant to bypass ActiveRecord for things that it can not do. And even for things that ActiveRecord doesn't support, you can always see if you can use the Squeel or Valium gems to handle edge-cases.
Another reason not to use find_by_sql is that, for example, using MySQL specific terms will lock you out of using other databases in the future.
In Ruby on rails 3 I want to query on a has_many field of a model as follows:
#project.items.where(:status => 1)
The problem is I'm trying to get the exact opposite result than this. What i want is all items of #project where the status is not 1. Been looking for the answer to this for a while, anyone?
There are many ways to accomplish what you are trying to do, however, some are better than others. If you will always be searching for a hardcoded number (i.e. 1 in this case), then the following solution will work:
#project.items.where('status != 1')
However, if this value is not hard-coded, you are openly vulnerable to SQL injection as Rails will not (cannot) escape this kind of query. As a result, it is preferred among Rails developers to user the following syntax for most custom conditions (those that can't be constructed via Hash):
#project.items.where(['status != ?', 1])
This syntax is slightly confusing, so let me go over it. Basically you are providing the where clause an Array of values. The first value in the array is a String representing the query you want executed. Anywhere you want a value in that string, you place a ?. This serves as a placeholder. Next, you add an element for every question mark in you query. For example, if I had the following:
where(['first_name = ? AND last_name = ?', params[:first_name], params[:last_name]]
Rails will automatically match these up forming the query for you. In that process, it also escapes potentially unsafe characters, preventing injection.
In general, it is preferred to use the Array syntax, even for a hardcoded value. I've been told that pure string conditions in Rails 3.5 will raise a warning (unverified), so it doesn't hurt to get in the process of using the Array syntax now.