I'm trying to model the following situation: A program has many versions, and one of the versions is the current one (not necessarily the latest).
This is how I'm doing it now:
class Program(Base):
__tablename__ = 'programs'
id = Column(Integer, primary_key=True)
name = Column(String)
current_version_id = Column(Integer, ForeignKey('program_versions.id'))
current_version = relationship('ProgramVersion', foreign_keys=[current_version_id])
versions = relationship('ProgramVersion', order_by='ProgramVersion.id', back_populates='program')
class ProgramVersion(Base):
__tablename__ = 'program_versions'
id = Column(Integer, primary_key=True)
program_id = Column(Integer, ForeignKey('programs.id'))
timestamp = Column(DateTime, default=datetime.datetime.utcnow)
program = relationship('Filter', foreign_keys=[program_id], back_populates='versions')
But then I get the error: Could not determine join condition between parent/child tables on relationship Program.versions - there are multiple foreign key paths linking the tables. Specify the 'foreign_keys' argument, providing a list of those columns which should be counted as containing a foreign key reference to the parent table.
But what foreign key should I provide for the 'Program.versions' relationship? Is there a better way to model this situation?
Circular dependency like that is a perfectly valid solution to this problem.
To fix your foreign keys problem, you need to explicitly provide the foreign_keys argument.
class Program(Base):
...
current_version = relationship('ProgramVersion', foreign_keys=current_version_id, ...)
versions = relationship('ProgramVersion', foreign_keys="ProgramVersion.program_id", ...)
class ProgramVersion(Base):
...
program = relationship('Filter', foreign_keys=program_id, ...)
You'll find that when you do a create_all(), SQLAlchemy has trouble creating the tables because each table has a foreign key that depends on a column in the other. SQLAlchemy provides a way to break this circular dependency by using an ALTER statement for one of the tables:
class Program(Base):
...
current_version_id = Column(Integer, ForeignKey('program_versions.id', use_alter=True, name="fk_program_current_version_id"))
...
Finally, you'll find that when you add a complete object graph to the session, SQLAlchemy has trouble issuing INSERT statements because each row has a value that depends on the yet-unknown primary key of the other. SQLAlchemy provides a way to break this circular dependency by issuing an UPDATE for one of the columns:
class Program(Base):
...
current_version = relationship('ProgramVersion', foreign_keys=current_version_id, post_update=True, ...)
...
This design is not ideal; by having two tables refer to one another, you cannot effectively insert into either table, because the foreign key required in the other will not exist. One possible solution in outlined in the selected answer of
this question related to microsoft sqlserver, but I will summarize/elaborate on it here.
A better way to model this might be to introduce a third table, VersionHistory, and eliminate your foreign key constraints on the other two tables.
class VersionHistory(Base):
__tablename__ = 'version_history'
program_id = Column(Integer, ForeignKey('programs.id'), primary_key=True)
version_id = Column(Integer, ForeignKey('program_version.id'), primary_key=True)
current = Column(Boolean, default=False)
# I'm not too familiar with SQLAlchemy, but I suspect that relationship
# information goes here somewhere
This eliminates the circular relationship you have created in your current implementation. You could then query this table by program, and receive all existing versions for the program, etc. Because of the composite primary key in this table, you could access any specific program/version combination. The addition of the current field to this table takes the burden of tracking currency off of the other two tables, although maintaining a single current version per program could require some trigger gymnastics.
HTH!
Related
I've got two models: User and Group.
User can be in one group so:
class User(db.Model):
# other fields
group_id = db.Column(db.Integer(), db.ForeignKey('group.id'))
but on the other hand I would also have some info about user who create that specific group:
class Group(db.Model):
# other fields
users = db.relationship("User", backref='group')
created_by = db.Column(db.Integer(), db.ForeignKey('user.id'))
Result is:
sqlalchemy.exc.CircularDependencyError: Can't sort tables for DROP; an unresolvable foreign key dependency exists between tables: group, user. Please ensure that the ForeignKey and ForeignKeyConstraint objects involved in the cycle have names so that they can be dropped using DROP CONSTRAINT.
I tried use_alter=True, but it gives me:
sqlalchemy.exc.CompileError: Can't emit DROP CONSTRAINT for constraint ForeignKeyConstraint(
Interestingly I'd expect you to get an AmbiguousForeignKeyError but instead you seem to get a CircularDependencyError? According to the docs this is caused by two scenarios:
In a Session flush operation, if two objects are mutually dependent on each other, they can not be inserted or deleted via INSERT or
DELETE statements alone; an UPDATE will be needed to post-associate or
pre-deassociate one of the foreign key constrained values. The
post_update flag described at Rows that point to themselves / Mutually
Dependent Rows can resolve this cycle.
In a MetaData.sorted_tables
operation, two ForeignKey or ForeignKeyConstraint objects mutually
refer to each other. Apply the use_alter=True flag to one or both, see
Creating/Dropping Foreign Key Constraints via ALTER.
I'm not sure what you're executing that's causing this particular error, but most likely you'll be able to solve it by solving the ambiguous reference.
The ambigious reference is due to SQLAlchemy not being able to figure out how to perform the join when there are multiple references (users and created_by in this case). This can be resolved by specifying how the relationship should join which can be done by either giving the specific foreign key it should use or by explicitly determining the join condition.
You can see these being applied to your example here:
class User(Base):
# Other setup / fields
group_id = Column(Integer, ForeignKey('group.id'))
class Group(Base):
# Other setup / fields
created_by_id = Column(Integer, ForeignKey('user.id'), nullable=False)
created_by = relationship("User", foreign_keys=[created_by_id])
users = relationship("User", backref="group", primaryjoin=id==User.group_id)
Documentation regarding relationship joins: http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#configuring-how-relationship-joins
I have an app I am building with Flask that contains models for Projects and Plates, where Plates have Project as a foreignkey.
Each project has a year, given as an integer (so 17 for 2017); and each plate has a number and a name, constructed from the plate.project.year and plate.number. For example, Plate 106 from a project done this year would have the name '17-0106'. I would like this name to be unique.
Here are my models:
class Project(Model):
__tablename__ = 'projects'
id = Column(Integer, primary_key=True)
name = Column(String(64),unique=True)
year = Column(Integer,default=datetime.now().year-2000)
class Plate(Model):
__tablename__ = 'plates'
id = Column(Integer, primary_key=True)
number = Column(Integer)
project_id = Column(Integer, ForeignKey('projects.id'))
project = relationship('Project',backref=backref('plates',cascade='all, delete-orphan'))
#property
def name(self):
return str(self.project.year) + '-' + str(self.number).zfill(4)
My first idea was to make the number unique amongst the plates that have the same project.year attribute, so I have tried variations on
__table_args__ = (UniqueConstraint('project.year', 'number', name='_year_number_uc'),), but this needs to access the other table.
Is there a way to do this in the database? Or, failing that, an __init__ method that checks for uniqueness of either the number/project.year combination, or the name property?
There are multiple solutions to your problem. For example, you can de-normalize project.year-number combination and store it as a separate Plate field. Then you can put a unique key on it. The question is how you're going to maintain that value. The two obvious options are triggers (assuming your DB supports triggers and you're ok to use them) or sqla Events, see http://docs.sqlalchemy.org/en/latest/orm/events.html#
Both solutions won't emit an extra SELECT query. Which I believe is important for you.
your question is somewhat similar to Can SQLAlchemy events be used to update a denormalized data cache?
I have db that I cannot modify, it has two tables 'people' and 'relation'. The table 'people' has names, ids and the column parent (yes/no). The table 'relation' contains a foreign key 'people.id' for parent and a 'people.id' for its child. I want to join columns in the people table so I can
People.query.filter_by(id='id of the parent')
to get the name of the parent and it's childs. This is my code:
class People(db.model):
__tablename__ = 'people'
id = db.Column(db.integer(), primary_key=True
name = db.Column(db.String())
parent = db.Column(db.Integer()) ##0 for no 1 for yes
parent_id=db.relationship('Link',backref=db.backref('Link.parent_id')
class Link(db.Model):
_tablename__ = 'link'
parent_id=db.Column(db.Integer(),db.ForeignKey('people.id'),primary_key=True)
id = db.Column(db.Integer(), db.ForeignKey('people.id'), primary_key=True)
dateofbirth = db.Column(db.Integer())
SQLAlchemy tells me:
ArgumentError: relationship 'parent_id' expects a class or a mapper argument (received: <class 'sqlalchemy.sql.schema.Table'>)
Excuse me if I messed up, but it's my first question here (and also the first steps with SQLAlchemy)
Typically you would want to set up the foreign key and backref in the same table, like this:
class Link(db.Model):
_tablename__ = 'link'
parent_id = db.Column(db.Integer(),db.ForeignKey('people.id'),primary_key=True)
parent = db.relationship('People', backref='links')
Now you can access each Link entries parent via Link.parent, and you can get a list of each People entries links via People.links (assuming this is a one-to-many relationship).
Also, if People.parent is supposed to represent a boolean value then:
1.) you should follow the standard naming convention and call it something like is_parent
2.) you should declare People.parent as a db.Boolean type, not a db.Integer. In most (probably all) database implementations, using booleans instead of integers (when appropriate) is more memory efficient.
I hope this helped.
This question essentially two parts.
1. I have a situation where I require things to be unique together i.e the elements in db need to be unique together with each other.
Lets say we have a model Things ( Rough PseudoCode)
Class ShoppingList( object ):
thing1_id = Column(Integer)
thing2_id = Column(Integer)
Now I need thing1_id and thing2_id to be a unique together ie the set of thing1_id and thing2_id needs to be unique together. Coming from django world I know that you can do a meta declaration in django models of unique_together. But how can do this in turbogears .
Also how do I actually apply a unique_together on a legacy system.
You simply want to add a UniqueConstraint to your table definition (using a primary key would achive similar effects, but with different semantics nevertheless).
This is as simple as:
Class ShoppingList( object ):
thing1_id = Column(Integer)
thing2_id = Column(Integer)
__table_args__ = (
UniqueConstraint('thing1_id', 'thing2_id'),
)
See also https://docs.sqlalchemy.org/en/latest/orm/extensions/declarative/table_config.html#table-configuration
For the first part of your question, if I understand your question correctly, I believe you are talking about the need for defining composite primary keys. As stated in http://docs.sqlalchemy.org/en/latest/core/schema.html#describing-databases-with-metadata:
Multiple columns may be assigned the primary_key=True flag which denotes a multi-column primary key, known as a composite primary key.
Defining such a relationship on a class using the declarative ORM way in SQLAlchemy, should be as simple as:
class ShoppingList(Base):
thing1_id = Column(Integer, primary_key=True)
thing2_id = Column(Integer, primary_key=True)
As for the second part of your question, I believe you mean how one would define the same SQLAlchemy mapping for an existing, legacy database. If so, you should be able to use the above approach, just don't create the database from the ORM definition. You may also use the classic mapping way, described in: http://docs.sqlalchemy.org/en/rel_0_8/orm/mapper_config.html?highlight=composite%20primary%20key#classical-mappings
I'm using the ORM side of SQLAlchemy, and I've defined one of my columns to have a foreign key relation to another model, using:
Base = declarative_base()
class Model1(Base):
__tablename__ = 'm1'
Name = Column(String, primary_key = True)
info = Column(String)
class Model2(Base):
__tablename__ = 'm2'
Name = Column(String, primary_key = True)
info = Column(String)
other_model = Column(String, ForeignKey('m1.Name'))
However, it doesn't seem to matter what I put in the other_model attribute, it seems more than happy to commit it to the database, even if there is no Model1 instance that has that Name.
It looks like the answer was in the database I was using (SQLite), not SQLAlchemy. SQLite versions <3.6.1 (AFAIK) do not support foreign key constraints.
The answer is therefore very similar to this answer on foreign keys and SQLAlchemy.
As I'm using Windows, I was able to go to the pysqlite2 page, the packaged installers have version 3.7.6.2 sqlite, and then and the final implementation was aided by this SQLAlchemy page on sqlite engines and dialects. This SO question is also relevant with regards to the upgrade process.
Finally, the SQLite engine is a bit temperamental when deciding whether or not to enforce the foreign key constraint, and this SO question is quite useful in forcing the foreign key enforcement.