SQLAlchemy多表和外键联接

Question

Got four tables; users , company , company_branch and users_branch . Users are people belonging to company. A company has got branches and a user can belong to a single branch at any given time. However, the users_branch table exists to keep track of the history of changing from one branch to another. Eg To get the current branch of a user with id 1, one would run a SELECT company_id, company_branch_id FROM users_branch WHERE user_id = 1 ORDER BY created_at DESC LIMIT 1 .

The challange I have is that am not able to figure out the correct not SQLAlchemy ORM syntax but also SQL raw to get list of users in a certain company at a given time and do so while returning the users_id, users_email_address, company_id, company_name, compancy_branch_id and company_branch_name for each entry. The queries I've tried so far either return nothing or they return repeated values in the users_branch wheareas I only want the latest branch for each user

Here is the link to the sqlfiddle sample postgresql database. In SQAlchemy the models are Users, Company, CompanyBranch, UsersBranch as seen below:

class Users(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    email_address = Column(String(70), nullable=False, unique=True)

class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)
    created_at = Column(DateTime, server_default=text('NOW()'), nullable=False)
    created_by = Column(ForeignKey('users.id'), nullable=False)
    company_name = Column(String(100), nullable=False, unique=True)

class CompanyBranch(Base):
    __tablename__ = 'company_branch'
    id = Column(Integer, primary_key=True)
    created_at = Column(DateTime, server_default=text('NOW()'), nullable=False)
    created_by = Column(ForeignKey('users.id'), nullable=False)
    company_id = Column(ForeignKey('company.id'), nullable=False)
    branch_name = Column(String(100), nullable=False, unique=True)

class UsersBranch(Base):
    __tablename__ = 'users_branch'
    id = Column(Integer, primary_key=True)
    created_at = Column(DateTime, server_default=text('NOW()'), nullable=False)
    created_by = Column(ForeignKey('users.id'), nullable=False)
    user_id = Column(ForeignKey('users.id'), nullable=False)
    company_id = Column(ForeignKey('company.id'), nullable=False)
    company_branch_id = Column(ForeignKey('company_branch.id'), nullable=False)

Answer 1

First, let me start by saying your schema is a little denormalized. users_branch.company_id is not necessary because users_branch.company_branch_id can also give you the company_id . There may be a good reason for it, but it might be adding some confusion here.

This is tricky because of the users_branch table. It essentially requires grouping by user_id and selecting the row with the max created_at .

SELECT DISTINCT ON (users_branch.user_id)
  *
FROM
  users
  JOIN users_branch ON users.id = users_branch.user_id
  JOIN company_branch ON users_branch.company_branch_id = company_branch.id
  JOIN company ON company_branch.company_id = company.id
WHERE users_branch.created_at < [some date]
ORDER BY users_branch.user_id, users_branch.created_at DESC;

This doesn't map well to the SQLAlchemy ORM, though.

Answer 2

I think I've nailed what I needed. The following raw SQL code seems to give me the proper answer of returning only the current branch where user's are. Took me a while but I also figured out the SQlAlchemy equivalent. I'll leave it here as an answer for a while and see if anyone else can tune it further.

Raw SQL

SELECT DISTINCT ON (users_branch.user_id) users.email_address, company.id as company_id, company.company_name, company_branch.id AS company_branch_id, company_branch.branch_name
FROM
  users
  JOIN users_branch ON users.id = users_branch.user_id
  JOIN company_branch ON users_branch.company_branch_id = company_branch.id
  JOIN company ON company_branch.company_id = company.id
WHERE users_branch.created_at in (SELECT max(users_branch.created_at) FROM users_branch GROUP BY users_branch.user_id) AND 
users_branch.company_id = 1 AND
users_branch.company_branch_id = 3

SQL Alchemy

query = session.query(Users.id.label('user_id'), Users.email_address, Company.id.label('company_id'), Company.company_name,
CompanyBranch.id.label("company_branch_id"), CompanyBranch.branch_name).distinct(UsersBranch.user_id). \
join(UsersBranch, and_(Users.id == UsersBranch.user_id)). \
join(CompanyBranch, and_(UsersBranch.company_branch_id == CompanyBranch.id)).\
join(Company, and_(CompanyBranch.company_id == Company.id)).\
filter(UsersBranch.created_at.in_(session.query(func.max(UsersBranch.created_at)).group_by(UsersBranch.user_id))).\
filter(UsersBranch.company_id == 1).\
filter(UsersBranch.company_branch_id == 3)