Pydantic Classes Generator

This code generator produces Pydantic classes, which represent the entities and relationships of a B-UML model. These Pydantic classes can be utilized by other code generators to generate code that uses Pydantic classes, such as REST API Generator and Backend Generator.

Let’s generate the code for the Pydantic classes of our Structural model example B-UML model example. You should create a PydanticGenerator object, provide the B-UML model, and use the generate method as follows:

from besser.generators.pydantic_classes import PydanticGenerator

generator: PydanticGenerator = PydanticGenerator(model=library_model)
generator.generate()

Upon executing this code, a pydantic_classes.py file containing the Pydantic models will be generated in the <<current_directory>>/output folder and it will look as follows.

from datetime import datetime, date
from typing import List, Optional, Union, Set
from enum import Enum
from pydantic import BaseModel


############################################
# Enumerations are defined here
############################################

############################################
# Classes are defined here
############################################
class AuthorCreate(BaseModel):
    name: str
    email: str
    publishes: List[int]

class LibraryCreate(BaseModel):
    address: str
    name: str

class BookCreate(BaseModel):
    title: str
    pages: int
    release: date
    writtenBy: List[int]
    locatedIn: int  # N:1 Relationship

OCL Constraint Validation

The Pydantic generator automatically generates field validators from OCL (Object Constraint Language) invariant constraints defined in your B-UML model. This provides automatic validation of data at the API level.

Defining OCL Constraints

You can define OCL constraints on your domain model classes:

from besser.BUML.metamodel.structural import Class, Constraint

Player = Class(name="Player", attributes={...})

age_constraint = Constraint(
    name="min_age",
    context=Player,
    expression="context Player inv: self.age > 10",
    language="OCL"
)

domain_model.constraints = {age_constraint}

Supported Operators

The following OCL comparison operators are supported:

OCL Operator	Python Equivalent	Example
>	>	self.age > 18
<	<	self.age < 65
>=	>=	self.score >= 0
<=	<=	self.price <= 100
=	==	self.status = 'active'
<>	!=	self.name <> ''

Generated Validators

For each OCL constraint, the generator produces a Pydantic field_validator:

class PlayerCreate(BaseModel):
    age: int
    name: str

    @field_validator('age')
    @classmethod
    def validate_age_1(cls, v):
        """OCL Constraint: min_age"""
        if not (v > 10):
            raise ValueError('age must be > 10')
        return v

These validators automatically enforce constraints when creating or updating entities via the REST API, and the error messages are displayed in the frontend web application.