Rails 3.1 - Using Scopes to Find Unenrolled Students - scope

# student.rb
has_and_belongs_to_many :courses
# course.rb
has_and_belongs_to_many :students
I'm trying to create a scope in the students model that will check if they are enrolled in a course.
The best I've come up with is:
scope :unenrolled, where(Student.courses.count => 0)
But then I get the error message
undefined method `courses'
Anybody offer any suggestions?

Alright then. So here's your code:
scope :unenrolled, where(Student.courses.count => 0)
The first problem here is the thing that's causing the error: You're calling the instance method courses on the class Student. As the name implies, you can only call an instance method on an instance of a class, not on the class itself. For example:
jim = Student.find(123)
jims_courses = jim.courses
But here's the kicker: When you call scope you're in the class context, i.e. the code isn't inside an instance method, so it gets called when your model is first declared. There's no instance at that time so you can't just call courses like you would from within one of Student's instance methods.
But that's kind of moot since you've slightly misunderstood how where works. The argument(s) you give to where are supposed to be conditions that correspond to what you would put after WHERE in an SQL query. For example where(:eye_color => 'brown') will be turned into an SQL WHERE clause like WHERE eye_color = 'brown'. :eye_color => 'brown' is just a Hash with the key :eye_color whose value is 'brown'. Calling a function on the left side of => doesn't make sense unless the function returns the name of a column/attribute in your model that ActiveRecord will understand.
So now let's figure out what you should do. If you were writing an SQL query it would look something like this:
SELECT `students`.*, COUNT(`courses_students`.*) AS `courses_count`
FROM `students`
JOIN `courses_students` ON `students`.`id` = `courses_students`.`student_id`
WHERE `courses_count` = '0'
GROUP BY `courses_students`.`student_id`;
This translates roughly to an ActiveRecord query like this:
Student.joins(:courses). // AR automatically joins courses though courses_students
select('students., COUNT(courses.) AS courses_count').
where('courses_count = 0').
group('id')
And you can plunk that directly into your scope:
scope :unenrolled, joins(:courses).
select('students.*, COUNT(courses.*) AS courses_count').
where('courses_count = 0').
group('courses.course_id')
Note: These queries are a bit off-the-cuff and may require a bit of tweaking. The easiest way to build complicated ActiveRecord queries is by entering them directly into the Rails console until you get the results you want.
Hope that's helpful!

Related

Creating Node.js enum in code to match list of values in database

I have a list of valid values that I am storing in a data store. This list is about 20 items long now and will likely grow to around 100, maybe more.
I feel there are a variety of reasons it makes sense to store this in a data store rather than just storing in code. I want to be able to maintain the list and its metadata and make it accessible to other services, so it seems like a micro-service data store.
But in code, we want to make sure only values from the list are passed, and they can typically be hardcoded. So we would like to create an enum that can be used in code to ensure that valid values are passed.
I have created a simple node.js that can generate a JS file with the enum right from the data store. This could be regenerated anytime the file changes or maybe on a schedule. But sharing the enum file with any node.js applications that use it would not be trivial.
Has anyone done anything like this? Any reason why this would be a bad approach? Any feedback is welcome.
Piggy-backing off of this answer, which describes a way of creating an "enum" in JavaScript: you can grab the list of constants from your server (via an HTTP call) and then generate the enum in code, without the need for creating and loading a JavaScript source file.
Given that you have loaded your enumConstants from the back-end (here I hard-coded them):
const enumConstants = [
'FIRST',
'SECOND',
'THIRD'
];
const temp = {};
for (const constant of enumConstants) {
temp[constant] = constant;
}
const PlaceEnum = Object.freeze(temp);
console.log(PlaceEnum.FIRST);
// Or, in one line
const PlaceEnum2 = Object.freeze(enumConstants.reduce((o, c) => { o[c] = c; return o; }, {}));
console.log(PlaceEnum2.FIRST);
It is not ideal for code analysis or when using a smart editor, because the object is not explicitly defined and the editor will complain, but it will work.
Another approach is just to use an array and look for its members.
const members = ['first', 'second', 'third'...]
// then test for the members
members.indexOf('first') // 0
members.indexOf('third') // 2
members.indexOf('zero') // -1
members.indexOf('your_variable_to_test') // does it exist in the "enum"?
Any value that is >=0 will be a member of the list. -1 will not be a member. This doesn't "lock" the object like freeze (above) but I find it suffices for most of my similar scenarios.

How to avoid duplication of class instances in Python?

What is a good way to avoid duplication of a class instance when it is created using the __init__() function.
This question is a result of this issue.
Context (using employee class example):
Lets say I have an employee class: __init__(self,name,dept)
I also have a method, employee.info(self) that prints out name and dept of any employee.
However a user could just add an employee by calling a=employee(args..). They could do it multiple times using the same instance variable a, but different employee names.
This will cause issues if they try to print a.info(), as each time a different employee name will be printed.
Is there a better way to do this? I know it is possible to have the __init__() "pass" and define a new method to create an instance.
Expect results:
>>Adam=employee('marketing')
>>Adam.info()
>>Adam works in marketing.
OR
>>a=employee('Adam','marketing')
>>a=employee('Mary','marketing')
>>Error: employee instance with identifier "a" already exists.
>>Use employee.update() method to modify existing record.
Is there a cleaner way of doing it? (as you might guess, I am still learning python).
Or is it good practice to write an explicit function (instead of a class method) to add new employees?
what you try is impossible, because in
a=employee('Adam','marketing')
a is not an object but a variable that points to the object employee('Adam','marketing').
When you do
a=employee('Mary','marketing')
you say to python that now, a must now not point to the object employee('Adam','marketing') but to the object employee('Mary','marketing'). And then, if you have no more variable to reference the object employee('Adam','marketing'), the garbage collector will destroy it.
You must consider that in python all is object, but not the variables that are only references to manipulate objects.
I have been racking my brains over the same problem and have finally managed to figure out a workaround :
Create a dictionary that stores the employee name and the related object like this :
total_emp_dict = {}
Add this inside the def __init__ of the class employee : total_emp_dict[name] = self. This will ensure to add each employee name as key and the object associated will be added as value.
Now create a new function outside & before the employee class is defined, let's call it create_new_emp. It will go like this :
#function to check and avoid duplicate assignment of instances
def create_new_emp(name, dept):
if name in total_emp_dict:
return total_emp_dict[name]
else:
return employee(name, dept)
When creating a any new employee, use this new function : a = create_new_emp("Adam", HR) instead of a = employee("Adam", HR)
Explanation : This function will ensure that duplicate assignment is not done. If "a" is already assigned to "Adam", this function will return object of "Adam" to "a", when called again. But if nothing is assigned to "a", then this function will handover to the attributes (name, dept) to employee class and create a new instance. This instance will then be assigned to "a".
I don't know if this is the best solution for this problem, but honestly this is the only solution I have found so far and it works great for me without much fuss / extra code space. Hope it works for you too! :)

Flask-AppBuilder equivalent of SQLite WHERE clause to filter column data

I'm new to Flask and have started designing a front end for an inventory management database using Flask-AppBuilder.
I have created several models and have have managed to display my sqlite data in tables using Flask-AppBuilder's views.
However, I don't seem to be able to find the equivalent of SQLite WHERE clause to filter or "restrict" column data. I've been reading a lot about sqlalchemy, filters, queries but this has left me more confused that anything else and the explanations seem to be extremely elaborate and complicated to do something which is extremely simple.
Assuming we reproduce the following SQLite query in Flask-AppBuilder:
SELECT Field_A
FROM Table_A
WHERE Field_A = 'some text'
with:
result = session.query(Table_A).filter_by(Field_A = 'some text').all()
Where does the above line of code go in my app?
Considering I have the following Class:
class Table_A(Model):
id = Column(Integer, primary_key=True)
Field_A = Column(String)
def __repr__(self):
return self
and View:
class Table_AView(ModelView):
datamodel = SQLAInterface(Table_AView)
label_columns = {'Field_A':'A'}
list_columns = ['Field_A']
After much digging flask-appbuilder uses it's own filterclass in order to enable you to filter your views.
All the classes are referenced here on GitHub:
Flask Filter Clases List
Also not the difference between FilterEqual and FilterEqualFunction here:
What is the difference between : FilterEqual and FilterEqualFunction?
For other customisation and first port of call of Flask-appbuilder go straight to the API Reference where you'll find a couple of examples of the filterclass in action.
In essence it is extremely simple. In your views.py code within the ModelView class you want to filter simply add base_filters = [['field_A', FilterEqual, 'abc']] like so:
`class Table_AView(ModelView):
datamodel = SQLAInterface(Table_AView)
label_columns = {'Field_A':'A'}
list_columns = ['Field_A']
base_filters = [['field_A', FilterEqual, 'abc']]`
This will only show the lines where the field_A variable is equal to abc.
Hope this helps someone as it took me nearly (sigh) two weeks to figure it out...
SQLALchemy is an ORM (Object-Relational Mapping), it mean that you dont have to deal with raw SQL, you will call a function that you "build" (by adding filters in your case). It will transparently generate an SQL query, execute it, and return the result as python objects.
I would suggest you to read closely at sqlalchemy documentation about filters again, especially filter_by :
http://docs.sqlalchemy.org/en/latest/orm/query.html#sqlalchemy.orm.query.Query.filter_by
It is the easiest way to apply a WHERE with sqlalchemy.
If you have declared correctly the model for Table_A, you should be able to use it so:
result = session.query(Table_A).filter_by(Field_A = 'some text').all()
Here session.query(Table_A).filter_by(Field_A = 'some text') will generate the SQL, and .all() will execute it.

How to convert Rep[T] to T in slick 3.0?

I used a code, generated from slick code generator.
My table has more than 22 columns, hence it uses HList
It generates 1 type and 1 function:
type AccountRow
def AccountRow(uuid: java.util.UUID, providerid: String, email: Option[String], ...):AccountRow
How do I write compiled insert code from generated code?
I tried this:
val insertAccountQueryCompiled = {
def q(uuid:Rep[UUID], providerId:Rep[String], email:Rep[Option[String]], ...) = Account += AccountRow(uuid, providerId, email, ...)
Compiled(q _)
}
I need to convert Rep[T] to T for AccountRow function to work. How do I do that?
Thank you
;TLDR; Not possible
Explanation
There are two levels of abstraction in Slick: Querys and DBIOActions.
When you're dealing with Querys, you have to access your schema definitions, and rows, Reps and, basically, it's very constrained as it's the closest level of abstraction to the actual DB you're using. A Rep refers to an hypothetical value in the database, not in your program.
Then you have DBIOActions, which are the next level... not just some definition of a query, but the execution of it. You usually get DBIOActions when getting information out of a query, like with the result method or (TADAN!) when inserting rows.
Inserts and Updates are not queries and so what you're trying to do is not possible. You're dealing with DBIOAction (the += method), and Query stuff (the Rep types). The only way to get a Rep inside a DBIOAction is by executing a Query and obtaining a DBIOAction and then composing both Actions using flatMap or for comprehensions (which is the same).

Retrieving Properties from DbSqlQuery

Background: Project is a Data Import utility for importing data from tsv files into a EF5 DB through DbContext.
Problem: I need to do a lookup for ForeignKeys while doing the import. I have a way to do that but the retrieval if the ID is not functioning.
So I have a TSV file example will be
Code Name MyFKTableId
codevalue namevalue select * from MyFKTable where Code = 'SE'
So when I process the file and Find a '...Id' column I know I need to do a lookup to find the FK The '...' is always the entity type so this is super simple. The problem I have is that I don't have access to the properties of the results of foundEntity
string childEntity = column.Substring(0, column.Length - 2);
DbEntityEntry recordType = myContext.Entry(childEntity.GetEntityOfReflectedType());
DbSqlQuery foundEntity = myContext.Set(recordType.Entity.GetType()).SqlQuery(dr[column])
Any suggestion would be appreciated. I need to keep this generic so we can't use known type casting. The Id Property accessible from IBaseEntity so I can cast that, but all other entity types must be not be fixed
Note: The SQL in the MyFKTableId value is not a requirement. If there is a better option allowing to get away from SqlQuery() I would be open to suggestions.
SOLVED:
Ok What I did was create a Class called IdClass that only has a Guid Property for Id. Modified my sql to only return the Id. Then implemented the SqlQuery(sql) call on the Database rather than the Set([Type]).SqlQuery(sql) like so.
IdClass x = ImportFactory.AuthoringContext.Database.SqlQuery<IdClass>(sql).FirstOrDefault();
SOLVED:
Ok What I did was create a Class called IdClass that only has a Guid Property for Id. Modified my sql to only return the Id. Then implemented the SqlQuery(sql) call on the Database rather than the Set([Type]).SqlQuery(sql) like so.
IdClass x = ImportFactory.AuthoringContext.Database.SqlQuery<IdClass>(sql).FirstOrDefault();

Resources