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Example: Understanding inheritance and relationships between model classes #488
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You need to use mapping (https://docs.sqlalchemy.org/en/14/orm/inheritance.html) to achieve this, but it's a bit tricky with sqlmodel.
Example of working code: from typing import Optional, List
from sqlmodel import SQLModel, Field, Relationship, create_engine, Session
from sqlalchemy.orm import registry, with_polymorphic
mapper_registry = registry()
class Contract(SQLModel, table=True):
"""A contract defines the business conditions of a project"""
id: Optional[int] = Field(default=None, primary_key=True)
title: str = Field(description="Short description of the contract.")
client: "Client" = Relationship(
back_populates="contracts",
)
client_id: Optional[int] = Field(
default=None,
foreign_key="client.id",
)
currency: str
term_of_payment: Optional[int] = Field(
description="How many days after receipt of invoice this invoice is due.",
default=31,
)
contact_type: str
__mapper_args__ = {
"polymorphic_identity": "contract",
"polymorphic_on": "contact_type",
}
@mapper_registry.mapped
class TimeContract(Contract, table=True):
"""A time-based contract with a rate per time unit"""
contract_id: Optional[int] = Field(
default=None, foreign_key="contract.id", primary_key=True
)
rate: float = Field(
description="Rate of remuneration",
)
unit: str = Field(
description="Unit of time tracked. The rate applies to this unit.",
default="hour",
)
__mapper_args__ = {
"polymorphic_identity": "time",
}
@mapper_registry.mapped
class WorksContract(Contract, table=True):
"""A contract with a fixed price"""
contract_id: Optional[int] = Field(
default=None, foreign_key="contract.id", primary_key=True
)
price: float = Field(
description="Price of the contract",
)
deliverable: str = Field(description="Description of the deliverable")
__mapper_args__ = {
"polymorphic_identity": "works",
}
class Client(SQLModel, table=True):
"""A client the freelancer has contracted with."""
id: Optional[int] = Field(default=None, primary_key=True)
name: str
contracts: List["Contract"] = Relationship(back_populates="client")
engine = create_engine(
"sqlite:///",
# echo=True,
)
with Session(engine) as session:
SQLModel.metadata.create_all(engine)
c = Client()
c.name = "client name"
c.contracts = [
TimeContract(currency=12.2, title="title1", term_of_payment=1, rate=3.4),
WorksContract(
title="title2",
currency=43.4,
term_of_payment=6,
price=344.2,
deliverable="---",
),
TimeContract(currency=13.2, title="title3", term_of_payment=12, rate=56.4),
]
session.add(c)
session.commit()
session.refresh(c)
contract_manager = with_polymorphic(Contract, [TimeContract, WorksContract])
query = session.query(contract_manager)
for i in query.all():
print(i) Edit: Maybe we can fix the problem mentioned in 2 with this: (main.py:292) for k, v in new_cls.__fields__.items():
if isinstance(getattr(new_cls, k, None), InstrumentedAttribute):
continue
col = get_column_from_field(v) |
Thanks @meirdev for the code example, I will try that.
If I do not care about minimizing the size of the database, is there still a problem with this? |
Would it be too much to ask for a framework like SQLModel to abstract away these parts of the code so that it's straightforward to use inheritance in models? Perhaps some black Python magic could add this in the background when subclasses are created.
From a user perspective, querying by |
I think this is ok, and it looks like sqlalchemy updates both tables each time. t = session.get(TimeContract, 1)
t.title = "new title"
session.add(t)
session.commit()
All these configurations give you flexibility to manipulate and query your data, for example: if you want to know how much contract time you have, you don't need to use session.query(TimeContract.id).count() |
SQLModel thinks the fields of the parent table are in the child table. This currently makes it unusable to use with joined table inheritance. This workaround proposed here with Ideally, joined table inheritance should work out of the box in SQLModel. In the meantime, does anyone have a fully working example? |
Hello, |
Same issue here |
Inheritance support would be really nice |
Is this really an inheritance issue? Looks more like an import issue. |
I would like to suggest an enhancement for SQLModel to support single table inheritance, similar to what is available in SQLAlchemy. This feature would allow us to define a class hierarchy on a single database table, which can be extremely useful for scenarios where different types of entities share common attributes but also have their unique fields. It would greatly enhance the flexibility and power of SQLModel. Here's the SQLAlchemy documentation for reference: Single Table Inheritance. |
First Check
Commit to Help
Example Code
Description
I would like to understand inheritance better using the following example:
The code example is my first attempt at implementing this. However, It is not yet correct:
Questions:
Contract
is related to a client. That makes me thinkclient
should be a member of theContract
base class. However, I don't understand how to properly implement the relationship. Is it necessary to moveclient
to the table classes (thereby duplicating it)?select
fromContract
(rather than from the different contract types separately)?Operating System
macOS, Other
Operating System Details
No response
SQLModel Version
0.0.8
Python Version
3.10
Additional Context
No response
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