I have two tables that look like this:
Products: id category name description active
Sales_sheets: id product_id link
product_id is a foreign key from the products id table
I wrote a prepared statement JOIN like this which works:
SELECT p.name, p.description, s.link FROM products AS p
INNER JOIN sales_sheets AS s ON p.id = s.product_id WHERE active=1 AND category=?
Basically a product can have a link to a PDF, but not every product will have a sales sheet. So if i try to bring up a product which doesn't have a sales sheet attached to it then it always returns no rows.
So i thought I'd have to use a LEFT OUTER JOIN in place of the INNER JOIN, but that returns no rows too, am I naming the tables in the wrong order? I've never had to use an OUTER join before?
SELECT p.name, p.description, s.link FROM products p
LEFT JOIN sales_sheets s ON p.id = s.product_id
WHERE active = 1 && category = ?
Related
I have two tables users and posts and they have association of has_many. I want to fetch details of both users and posts in a single query. I'm able to manage the sql query but I don't want to use the raw query in the code (using execute method) as i think it is kind of simple thing and can be written using active record.
Here is the sql query
SELECT a.id, a.name, a.timestamp, b.id, b.user_id, b.title
FROM users a
INNER JOIN (SELECT id, user_id, title, from, to FROM posts) b on b.user_id = a.id
where id IN ( 1, 2, 3);
I think includes does not help here because i'm dealing with large data.
Can any one help me ?
If you just want those specific columns and nothing else then this will work
User.joins(:post)
.where(id: [1,2,3])
.select("users.id, users.name, users.timestamp,
posts.id as post_id, posts.user_id as post_user_id,
posts.title as post_title")
This will return an ActiveRecord::Relation of User objects with virtual attributes for post_id, post_user_id (Not sure why you need this one since you already selected users.id), and post_title.
The query produced will be
SELECT users.id,
users.name,
users.timestamp,
posts.id as post_id,
posts.user_id as post_user_id,
posts.title as post_title
FROM users
INNER JOIN posts on posts.user_id = users.id
where users.id IN ( 1, 2, 3);
Please note you may have multiple User objects, one for each Post, just as the SQL query does.
You can execute your exact query using the string version of joins e.g.
User.joins("INNER JOIN (SELECT id, user_id, title, from, to FROM posts) b on b.user_id = users.id")
.where(id: [1,2,3])
.select("users.id, users.name, users.timestamp,
b.id as post_id, b.user_id as post_user_id,
b.title as post_title")
Additionally to avoid some of the overhead you can use arel instead e.g.
users_table = User.arel_table
posts_table = Post.arel_table
query = users_table.project(Arel.star)
.join(posts_table)
.on(posts_table[:user_id].eq(users_table[:id]))
.where(users_table[:id].in([1,2,3]))
ActiveRecord::Base.connection.exec_query(query.to_sql)
This will return an ActiveRecord::Result with 2 useful methods columns (the columns selected) and rows. You can convert this to a Hash(#to_hash) but note that any columns with duplicate names (id for instance) will overwrite one another.
You could fix this by specifying the colums you want selected in the project portion. e.g. your current query would be:
query = users_table.project(
users_table[:id],
users_table[:name],
users_table[:timestamp],
posts_table[:id].as('post_id'),
posts_table[:user_id].as('post_user_id'),
posts_table[:title].as('post_title')
).join(posts_table)
.on(posts_table[:user_id].eq(users_table[:id]))
.where(users_table[:id].in([1,2,3]))
ActiveRecord::Base.connection.exec_query(query.to_sql).to_hash
Since none of the names collide now it can be structured into a nice Hash where the keys are the column names and the values or the row value for that record.
users = User.joins(:posts).includes(:posts).where(id: [1, 2, 3])
Will give you all the users with theirs posts.
then you can do whatever you want with them, but to access posts data for first retrieved user
first_user_posts = users.first.posts # this will not make additional DB queries as you used includes and data is already added
We use joins to have INNER JOIN statement in the SQL
We use includes to load all posts in the memory
I have two tables users and posts and they have association of
has_many. I want to fetch details of both users and posts in a single
query.
can be done with includes like
users = User.includes(:posts).where({posts: {user_id: [1,2,3]}})
other is eager_load and preload you can use as per your requirements, for more https://blog.arkency.com/2013/12/rails4-preloading/
I am doing a join between tables to get a user pic from a user table about someone who comments on an article. However, I need to get a separate picture from the user table for a different user, the person who wrote the article at the same time. (There is a strong reason to do this in one query, as it's actually only one part of a more complex request).
Can anyone suggest how I could get two user pics from the users table during the join? This code gives me the first user pic, the commenter's pic or upic. But how do I also get the second user pic?
commments
id|comment|articleid|userid
1|great article|1|2
article
id|article|userid
1|Restaurant Openings|1
users
id|pic
1|mary.png
2|joe.png
$sql = "SELECT c.comment,c.articleid,c.userid,u.id, u.pic as upic,a.id,
FROM `comments` c
LEFT JOIN `articles` a
ON c.articleid = a.id
LEFT JOIN `users` u
ON c.userid= u.id
WHERE c.id=1";
A separate query to get the author's pic for would be:
$sqlauthorpic = "SELECT u.pic,u.id from `users` u
WHERE u.id = 1";
Thanks for any suggestions.
Join users again this time with article table, since you are looking for a user that wrote the article:
SELECT c.comment,c.articleid,c.userid,u.id, u.pic as upic,a.id, w.pic as wpic
FROM `comments` c
LEFT JOIN `articles` a ON c.articleid = a.id
LEFT JOIN `users` u ON c.userid= u.id
JOIN `users` w ON w.id = a.userid
You can also get rid of LEFT joins because, i think there could be no comments with out an article (articleid) and no comment without userid.
This is a hard problem to describe but I have Rails query where I join another table and I want to exclude any results where the join table contain one of three conditions.
I have a Device model that relates to a CarUserRole model/record. In that CarUserRole record it will contain one of three :role - "owner", "monitor", "driver". I want to return any results where there is no related CarUserRole record where role: "owner". How would I do that?
This was my first attempt -
Device.joins(:car_user_roles).where('car_user_roles.role = ? OR car_user_roles.role = ? AND car_user_roles.role != ?', 'monitor', 'driver', 'owner')
Here is the sql -
"SELECT \"cars\".* FROM \"cars\" INNER JOIN \"car_user_roles\" ON \"car_user_roles\".\"car_id\" = \"cars\".\"id\" WHERE (car_user_roles.role = 'monitor' OR car_user_roles.role = 'driver' AND car_user_roles.role != 'owner')"
Update
I should mention that a device sometimes has multiple CarUserRole records. A device can have an "owner" and a "driver" CarUserRole. I should also note that they can only have one owner.
Anwser
I ended up going with #Reub's solution via our chat -
where(CarUserRole.where("car_user_roles.car_id = cars.id").where(role: 'owner').exists.not)
Since the car_user_roles table can have multiple records with the same car_id, an inner join can result in the join table having multiple rows for each row in the cars table. So, for a car that has 3 records in the car_user_roles table (monitor, owner and driver), there will be 3 records in the join table (each record having a different role). Your query will filter out the row where the role is owner, but it will match the other two, resulting in that car being returned as a result of your query even though it has a record with role as 'owner'.
Lets first try to form an sql query for the result that you want. We can then convert this into a Rails query.
SELECT * FROM cars WHERE NOT EXISTS (SELECT id FROM car_user_roles WHERE role='owner' AND car_id = cars.id);
The above is sufficient if you want devices which do not have any car_user_role with role as 'owner'. But this can also give you devices which have no corresponding record in car_user_roles. If you want to ensure that the device has at least one record in car_user_roles, you can add the following to the above query.
AND EXISTS (SELECT id FROM car_user_roles WHERE role IN ('monitor', 'driver') AND car_id = cars.id);
Now, we need to convert this into a Rails query.
Device.where(
CarUserRole.where("car_user_roles.car_id = cars.id").where(role: 'owner').exists.not
).where(
CarUserRole.where("car_user_roles.car_id = cars.id").where(role: ['monitor', 'driver']).exists
).all
You could also try the following if your Rails version supports exists?:
Device.joins(:car_user_roles).exists?(role: ['monitor', 'driver']).exists?(role: 'owner').not.select('cars.*').distinct
Select the distinct cars
SELECT DISTINCT (cars.*) FROM cars
Use a LEFT JOIN to pull in the car_user_roles
LEFT JOIN car_user_roles ON cars.id = car_user_roles.car_id
Select only the cars that DO NOT contain an 'owner' car_user_role
WHERE NOT EXISTS(SELECT NULL FROM car_user_roles WHERE cars.id = car_user_roles.car_id AND car_user_roles.role = 'owner')
Select only the cars that DO contain either a 'driver' or 'monitor' car_user_role
AND (car_user_roles.role IN ('driver','monitor'))
Put it all together:
SELECT DISTINCT (cars.*) FROM cars LEFT JOIN car_user_roles ON cars.id = car_user_roles.car_id WHERE NOT EXISTS(SELECT NULL FROM car_user_roles WHERE cars.id = car_user_roles.car_id AND car_user_roles.role = 'owner') AND (car_user_roles.role IN ('driver','monitor'));
Edit:
Execute the query directly from Rails and return only the found object IDs
ActiveRecord::Base.connection.execute(sql).collect { |x| x['id'] }
Suppose we have a model Order and Item where order has_many items.
I'd now like to get the sum of all items for each order.
#orders = Order.select("orders.*, sum(items.amount) as items_sum)
.joins("LEFT JOIN orders ON items.order_id = orders.id")
.group("orders.id")
produces
SELECT sum(items.amount) as items_sum, transfers.* FROM "orders"
LEFT JOIN items ON items.order_id = orders.id GROUP BY orders.id
The SQL itself works fine. The problem is that #orders doesn't include the items_sum. I'd like to be able to do #orders[0].items_sum. How is that possible?
you should be able to access it as
#orders.first.items_sum
or
#orders.first["items_sum"]
Documentation here
I have a BlogPost model with a :category attribute. I am making a new BlogPost form, and I would like for the select menu to populate itself with each category entered in previous records by the user.
Therefore, I need one query to find all BlogPosts with a User ID, and then round up a list of each category they have entered. The same category will exist in multiple records, but of course I only want to return copy of it for the select menu.
Thank you :)
You can SELECT DISTINCT categories returned an INNER JOIN to the right user :
Category
.joins( :posts )
.where( posts: { user_id: current_user.id } )
.uniq
This should send a query like this :
SELECT DISTINCT categories.*
FROM categories
INNER JOIN posts ON posts.category_id = categories.id
WHERE posts.user_id = [whatever]
EDIT NOTE: be wary that uniq is both a method on a Relation and on an Array. Be sure to call it on the relation before it is cast to an array, or you will perform the uniq on an array of non-distinct results, which works too, but is absurd performance-wise.