When I try to join a many-to-many table and group it by the main-id I am getting duplicates when I add the second many-to-many table.

Here is how my models look like:

Models

user

class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_fistName = db.Column(db.String(64))
    ...

student_identifier

student_identifier = db.Table('student_identifier',
    db.Column('class_id', db.Integer, db.ForeignKey('class.class_id')),
    db.Column('id', db.Integer, db.ForeignKey('user.id'))
)

class

class Class(db.Model):
    sqlite_autoincrement=True
    class_id = db.Column(db.Integer, primary_key=True)
    class_name = db.Column(db.String(128), unique=True)
    mm_children = db.relationship('User', secondary=student_identifier, backref=db.backref('classes'))

class_course_identifier

class_course_identifier = db.Table('class_course_identifier',
    db.Column('course_id', db.Integer, db.ForeignKey('course.course_id')),
    db.Column('class_id', db.Integer, db.ForeignKey('class.class_id'))
)

database structure

Well I am using SQLAlchemy to select the desired tables with the data I want. with this session.query

db.session.query(
   Class.class_id,
   Class.class_name,
   func.group_concat(User.user_fistName),
   func.group_concat(Course.course_name)
   ).filter(Class.courses, User.classes).group_by(Class.class_id)

the problem is that I am getting duplicates of both the courses AND names, so if the course has two users it will print the students and the course two times. Here is how it is looking:

wrong view

And here is how it should look:

correct view

the problem

the problem is coming when I am adding the second many-to-many table, for example users/student-identifier. If I remove the line where I "join" it, I am getting the duplicates. Is there anyway to correct this? Or should I use RAW-SQL instead(and if yes, how?)

Found out the solution, and it is quite simple.

RAW SQL

SELECT
  class.class_id,
  class.class_name,
  GROUP_CONCAT(DISTINCT course.course_name),
  GROUP_CONCAT(DISTINCT user.user_fistName)  
FROM
  class
JOIN class_course_identifier ON class.class_id = class_course_identifier.class_id
JOIN course ON class_course_identifier.course_id = course.course_id
JOIN student_identifier ON class.class_id = student_identifier.class_id
JOIN user ON student_identifier.id = user.id
GROUP BY class.class_id

SQLAlchemy

db.session.query(
   Class.class_id,
   Class.class_name,
   func.group_concat(User.user_fistName.distinct()),
   func.group_concat(Course.course_name.distinct())
   ).filter(Class.courses, User.classes).group_by(Class.class_id)

Simply add the distinct() to the desired column you want to be unique

来源：https://stackoverflow.com/questions/26583832/sqlalchemy-group-concat-and-duplicates

标签

python