Users have_many posts.
In a view, I want to get an alphabetized list of all users that have at least 1 post. Here is how I currently do it. Is there a way to make this all one line / using better Rails ActiveRecord conventions?
#users.order("name ASC").each do |user|
if user.posts > 0
...
end
end
Your current solution isn't bad (it's a single query) but it can be improved.
You can use ActiveRecord's built-in counter cache functionality to store the number of associated objects on the parent (in this case, the number of posts associated with a user). Then you can craft a query like this:
User.where('posts_count > 0').order('name ASC')
Here are the docs on :counter_cache (taken from here):
:counter_cache
Caches the number of belonging objects on the associate class through the use of increment_counter and decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it's destroyed. This requires that a column named #{table_name}_count (such as comments_count for a belonging Comment class) is used on the associate class (such as a Post class) - that is the migration for #{table_name}_count is created on the associate class (such that Post.comments_count will return the count cached, see note below). You can also specify a custom counter cache column by providing a column name instead of a true/false value to this option (e.g., counter_cache: :my_custom_counter.) Note: Specifying a counter cache will add it to that model's list of readonly attributes using attr_readonly.
We can get all the user ids which have atleast one post using this.
Post.uniq.pluck(:user_id);
And then we can fetch all the user as follows.
User.order(:name).find(Post.uniq.pluck(:user_id));
User.joins(:posts).order('users.name asc') will perform an inner join, as described in the documentation here. A counter cache isn't a bad solution either.
Related
I'm looking for the best Ruby way to accomplish this.
I have a Person that has_many Feeds through Subscriptions. So we can do things like Person.feeds, and it gets all the feeds a person is subscribed to.
Problem is, subscriptions are either authorized or deauthorized. What is the best way to make Person.feeds respect the Authorized status bit on the Subscription model?
So we can do something like Person.feeds.where(:status => authorized).
You can call this with a command like the following:
#person.feeds.joins(:subscription).where(subscriptions: { status: 'authorized' })
N.B. the joins takes the association's format, singular in this case, while where takes the table name, typically pluralised.
What this does in order is:
Loads the feeds belonging to a person
Joins these feeds to their subscription
Queries the subscriptions table to return only the feeds where the subscription is active
To refactor this, I'd include a couple of methods in the relevant models:
# feed.rb
scope :active, -> { joins(:subscription).where(subscriptions: { status: 'authorized' }) }
# person.rb
def active_feeds
feeds.active
end
Then, you can just call #person.active_feeds to get the results you want from anywhere in your code base.
(There's also the added bonus of Feed.active being available anywhere should you wish to display active feeds outside of a user's scope.)
Hope that helps - let me know if you've any questions.
I have a code section like the following:
users = User.all(:fname => "Paul")
This of course results in getting all users called "Paul". Now I only need some of the columns available for each user which leads to replacing the above line by something like this:
users = User.all(:name => "Paul", :fields => [:id, :fname, :lname, :email])
Until now everything works as expected. Unfortunately now I want to work with users but as soon as I use something like users.to_json, also the other columns available will be lazy-loaded even due the fact, that I don't need those. What's the correct or at least a good way to end up with users only containing the attributes for each user that I need?
An intermediate object like suggested in How to stop DataMapper from double query when limiting columns/fields? is not a very good option as I have a lot of places where would need to define at least twice which fields I need and also I would loose the speed improvement gained by loading only the needed data from the DB. In addition such an intermediate object also seems to be quite ugly to build when having multiple rows of the DB selected (=> multiple objects in a collection) instead of just one.
If you usually works with the collection using json I suggest overriding the as_json method in your model:
def as_json(options = nil)
# this example ignores the user's options
super({:only => [:fname]}.merge(options || {}))
end
You are able to find more detailed explanation here http://robots.thoughtbot.com/better-serialization-less-as-json
Taken from: https://github.com/inossidabile/protector/issues/10
I want to be able to set permissions based on a joining table.
so...
Post.restrict!(current_user).joins(:category)
In this situation the current user doesn't have direct access to category but can get category through Post. How would I accomplish this? It's applying the default category scope and I don't see a way to make it conditional based on the joining table.
Please read this carefully: https://github.com/inossidabile/protector#self-aware-conditions. As you might see, you can accept second parameter to the restriction block. There, inside, you can get any its property or any nested association. So in your case it could be something like:
protect do |user, post|
if post.try(:category) && post.category.anything == 'foobar'
# Whatever you want to allow or disallow here
end
end
There are two classes: User and Question
A user may have many questions, and it also contains a question_count
to record the the count of questions belong to him.
So, when I add a new question, I want update the question_count of the
user. At first, I do as:
question = Question(title='aaa', content='bbb')
Session.add(question)
Session.flush()
user = question.user
### user is not None
user.question_count += 1
Session.commit()
Everything goes well.
But I wan't to use event callback to do the same thing. As following:
from sqlalchemy.orm.interfaces import MapperExtension
class Callback(MapperExtension):
def after_insert(self, mapper, connection, instance):
user = instance.user
### user is None !!!
user.question_count += 1
class Question(Base):
__tablename__ = "questions"
__mapper_args__ = {'extension':Callback()}
....
Note in the "after_insert" method:
instance.user # -> Get None!!!
Why?
If I change that line to:
Session.query(User).filter_by(id=instance.user_id).one()
I can get the user successfully, But: the user can't be updated!
Look I have modified the user:
user.question_count += 1
But there is no 'update' sql printed in the console, and the
question_count are not updated.
I try to add Session.flush() or Session.commit() in the
after_insert() method, but both cause errors.
Is there any important thing I'm missing? Please help me, thank you
The author of sqlalchemy gave me an useful answer in a forum, I copy it here:
Additionally, a key concept of the
unit of work pattern is that it
organizes a full list of all
INSERT,UPDATE, and DELETE statements
which will be emitted, as well as the
order in which they are emitted,
before anything happens. When the
before_insert() and after_insert()
event hooks are called, this structure
has been determined, and cannot be
changed in any way. The
documentation for before_insert() and
before_update() mentions that the
flush plan cannot be affected at this
point - only individual attributes on
the object at hand, and those which
have not been inserted or updated yet,
can be affected here. Any scheme
which would like to change the flush
plan must use
SessionExtension.before_flush.
However, there are several ways of
accomplishing what you want here
without modifiying the flush plan.
The simplest is what I already
suggested. Use
MapperExtension.before_insert() on the
"User" class, and set
user.question_count =
len(user.questions). This assumes
that you are mutating the
user.questions collection, rather than
working with Question.user to
establish the relationship. If you
happened to be using a "dynamic"
relationship (which is not the case
here), you'd pull the history for
user.questions and count up what's
been appended and removed.
The next way, is to do pretty much
what you think you want here, that is
implement after_insert on Question,
but emit the UPDATE statement
yourself. That's why "connection" is
one of the arguments to the mapper
extension methods:
def after_insert(self, mapper, connection, instance):
connection.execute(users_table.update().\
values(question_count=users_table.c.question_count +1).\
where(users_table.c.id==instance.user_id))
I wouldn't prefer that approach since
it's quite wasteful for many new
Questions being added to a single
User. So yet another option, if
User.questions cannot be relied upon
and you'd like to avoid many ad-hoc
UPDATE statements, is to actually
affect the flush plan by using
SessionExtension.before_flush:
class
MySessionExtension(SessionExtension):
def before_flush(self, session, flush_context):
for obj in session.new:
if isinstance(obj, Question):
obj.user.question_count +=1
for obj in session.deleted:
if isinstance(obj, Question):
obj.user.question_count -= 1
To combine the "aggregate" approach of
the "before_flush" method with the
"emit the SQL yourself" approach of
the after_insert() method, you can
also use SessionExtension.after_flush,
to count everything up and emit a
single mass UPDATE statement with many
parameters. We're likely well in the
realm of overkill for this particular
situation, but I presented an example
of such a scheme at Pycon last year,
which you can see at
http://bitbucket.org/zzzeek/pycon2010/src/tip/chap5/sessionextension.py
.
And, as I tried, I found we should update the user.question_count in after_flush
user, being I assume a RelationshipProperty, is only populated after the flush (as it is only this point the ORM knows how to relate the two rows).
It looks like question_count is actually a derived property, being the number of Question rows for that user. If performance is not a concern, you could use a read-only property and let the mapper do the work:
#property
def question_count(self):
return len(self.questions)
Otherwise you're looking at implementing a trigger, either at the database-level or in python (which modifies the flush plan so is more complicated).
How do I tell the Zend_Form that I want an element (and it's ID-label, etc) to use another ID value instead of the element's name?
I have several forms in a page. Some of them have repeated names. So as Zend_Form creates elements' IDs using names I end up with multiple elements with the same ID, which makes my (X)HTML document invalid.
What is the best solution to fix this, given that I really have to stick with using the same element names (they are a hash common to all forms and using the Zend_Form Hash Element is really out of question)?
Zend_Form_Element has a method called setAttribs that takes an array. You may be able to do something like $element->setAttribs(array('id' => "some_id"));
or you can do $element->setAttrib('id', 'some_id');
Thanks, Chris Gutierrez.
However, as I said, I needed to get ride of the default decorator generated IDs like -label. Wiht the $element->setAttribs() it is not possible, however.
So based on http://framework.zend.com/issues/browse/ZF-7125 I just did the following:
$element->clearDecorators();
$element->setAttrib('id', 'some_id');
$element->addDecorator("ViewHelper");
Whoever sees this: please note this was enough for what I needed. But may not be for you (the default settings has more than the viewHelper decorator).