I added the eager-loading but it seems to slow down the SQL query quite a lot. It's as if now, the SQL query is taking longer but the generating of the file is quicker hehe... I guess now the queries are being fired before to populate subjects. It's still taking relatively the same amount of time though.
On Friday, 3 July 2020 17:07:43 UTC+2, Simon King wrote: > > Are you eager-loading the "student.subjects" relationship? If not, > that will give you the biggest performance increase. Without that, you > will be issuing a separate DB query for each of the students, to load > that student's subjects. Eager-loading allows you to preload the > subjects for every student in a single query: > > > https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html#joined-eager-loading > > > Simon > > On Fri, Jul 3, 2020 at 1:36 PM Justvuur <just...@gmail.com <javascript:>> > wrote: > > > > Hi Simon, thanks for the help! I've never used that before, it's quite > handy. > > > > I'm looping through all the students and printing them and their subject > details to a CSV file. > > What makes things a tad complicated is the subjects must appear in a > specific order. > > There is a table that has the subject code and order number > (ordered_subjects used below is the resultset from it). > > I printed out the timing and found the problem to be with a nested for > loop. > > > > I was hoping to reduce that process time by using a map that > automatically gets populated instead of having to create it on the fly. > > > > Before - subjects_collection "attribute_mapped_collection": > > > ******************************************************************************************** > > > > for row in students: > > row_no += 1 > > > > for subject in row.subjects: > > student_subjects[subject.code] = subject.value > > > > csv_row = [row_no] > > csv_row += [student_subjects.get(x.code, '') for x in > ordered_subjects] > > csv_row += [row.created_on, row.updated_on] > > > > writer.writerow([x.encode('utf-8') if type(x) == unicode else x for > x in csv_row]) > > > > > > After adding the subjects_collection "attribute_mapped_collection", I > unfortunately did not see a change in performance. > > > > After - subjects_collection "attribute_mapped_collection": > > > ******************************************************************************************** > > > > for row in students: > > row_no += 1 > > csv_row = [row_no] > > csv_row += [row.subjects_collection.get(x.code, '').value for x in > ordered_subjects] > > csv_row += [row.created_on, row.updated_on] > > > > writer.writerow([x.encode('utf-8') if type(x) == unicode else x for > x in csv_row]) > > > > > > class Subject(db.Model): > > __tablename__ = 'subjects' > > > > student_id = db.Column(db.Integer, db.ForeignKey('students.id'), > primary_key=True) > > > > code = db.Column(db.String(50), primary_key=True) > > > > value= db.Column(db.String) > > > > def __init__(self, code , value): > > self.code = code > > self.value = value > > > > > > class Student(ResourceMixin, db.Model): > > __tablename__ = 'students' > > > > subjects= db.relationship('Subject', backref='student') > > > > id = db.Column(db.Integer, primary_key=True) > > > > subjects_collection = relationship("Subject", > collection_class=attribute_mapped_collection('code')) > > > > Can you see a way I can optimize this? Any ideas? > > > > > > On Friday, 3 July 2020 12:31:03 UTC+2, Simon King wrote: > >> > >> Are you trying to optimise the database access (ie. minimize the > >> number of queries), or provide a nice dictionary-style API for your > >> Student objects? What do you mean when you say that looping over > >> student.subjects is quite heavy? > >> > >> An association proxy can be used to get dict-style access to a > relationship: > >> > >> > https://docs.sqlalchemy.org/en/13/orm/extensions/associationproxy.html#proxying-to-dictionary-based-collections > > >> > >> There are also a couple of examples in the SQLAlchemy docs that > >> provide a dictionary-style API: > >> > >> > https://docs.sqlalchemy.org/en/13/orm/examples.html#module-examples.dynamic_dict > > >> > >> > https://docs.sqlalchemy.org/en/13/orm/examples.html#module-examples.vertical > >> > >> Hope that helps, > >> > >> Simon > >> > >> On Thu, Jul 2, 2020 at 8:46 PM Justvuur <just...@gmail.com> wrote: > >> > > >> > Hi there, > >> > > >> > I'm struggling to find an efficient way to get a two columned subset > into dictionary form. > >> > > >> > I have an entity that has a subset of data. The subset is linked to > the entity via Id. The order of the subset of data is defined in another > table. > >> > > >> > Example: > >> > Student - Id, firstname, lastname > >> > Subjects - StudentId, SubjectCode, SubjectName > >> > > >> > At the moment I'm looping through the SqlAlchemy result of > "student.subjects" in python and creating a dictionary from that. It's > quite heavy, especially when there are 2000+ students with a potential of > 100+ subjects each. > >> > > >> > For each student, how do I get the subjects as a dictionary for a > student where the key is the SubjectCode and the value is the SubjectName? > >> > Better yet, how can I get a result set: Id, firstname, lastname > SubjectCode x, SubjectCode y, etc etc (where the SubjectName becomes the > value and the SubjectCode becomes the column)? > >> > > >> > Regards, > >> > Justin > >> > > >> > -- > >> > SQLAlchemy - > >> > The Python SQL Toolkit and Object Relational Mapper > >> > > >> > http://www.sqlalchemy.org/ > >> > > >> > To post example code, please provide an MCVE: Minimal, Complete, and > Verifiable Example. 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