Entity Class (ER Diagrams)

The Entity class represents a table or entity in an Entity Relationship (ER) diagram. Each entity contains a set of attributes that describe its properties.

Overview

An Entity is a logical representation of a table or concept in a database schema. It contains attributes (columns) with their types and optional constraints like Primary Key (PK), Foreign Key (FK), or Unique Key (UK).

Constructor

Entity(
    name: str,
    attributes: Optional[dict[str, Union[list[str], str]]] = None
)

Parameters

name (str): The name of the entity (typically the table name)
attributes (Optional[dict]): Dictionary mapping attribute names to their definitions. Values can be:
- A string: Just the type (e.g., "int", "string")
- A list: Type and constraints/comments (e.g., ["int", "PK"] or ["string", "comment"])

Attributes

After initialization:

name (str): The entity's name
attributes (dict): Dictionary of attributes

Methods

add_attribute(name: str, type_: str, constraint: Optional[str] = None, comment: Optional[str] = None) -> None

Add a single attribute to the entity with optional constraint and comment.

Parameters:

name (str): The attribute name
type_ (str): The data type (e.g., "int", "string", "datetime")
constraint (Optional[str]): Optional constraint like "PK", "FK", "UK", or combination like "FK PK"
comment (Optional[str]): Optional comment describing the attribute

Example:

entity = Entity("USER")
entity.add_attribute("user_id", "int", constraint="PK")
entity.add_attribute("email", "string", constraint="UK")
entity.add_attribute("created_at", "datetime", comment="Account creation timestamp")

update_attributes(attributes: dict[str, str]) -> None

Update multiple attributes at once.

Parameters:

attributes (dict): Dictionary of attribute names and their type definitions

Example:

entity.update_attributes({
    "username": "string",
    "password": "string",
    "is_active": "boolean"
})

Attribute Constraints

Use these standard database constraints:

Constraint	Meaning	Usage
`PK`	Primary Key	Uniquely identifies each record
`FK`	Foreign Key	References a primary key in another entity
`UK`	Unique Key	Ensures unique values
`PK FK`	Composite Key	Both primary and foreign key

Basic Examples

Simple Entity

from mermaid.erdiagram import Entity
 
product = Entity("PRODUCT", {
    "product_id": "int",
    "name": "string",
    "price": "decimal",
    "quantity": "int"
})

Entity with Constraints

user = Entity("USER")
user.add_attribute("user_id", "int", constraint="PK")
user.add_attribute("email", "string", constraint="UK")
user.add_attribute("username", "string", constraint="UK")
user.add_attribute("password", "string")
user.add_attribute("is_active", "boolean")

Entity with Comments

order = Entity("ORDER")
order.add_attribute("order_id", "int", constraint="PK")
order.add_attribute("customer_id", "int", constraint="FK", comment="References CUSTOMER table")
order.add_attribute("order_date", "datetime", comment="When the order was placed")
order.add_attribute("total_amount", "decimal", comment="Sum of all items in order")
order.add_attribute("status", "enum", comment="pending, processing, shipped, delivered, cancelled")

Entity with Multiple Constraints

enrollment = Entity("ENROLLMENT")
enrollment.add_attribute("student_id", "int", constraint="FK PK")
enrollment.add_attribute("course_id", "int", constraint="FK PK")
enrollment.add_attribute("grade", "char")
enrollment.add_attribute("enrollment_date", "datetime")

Attribute Data Types

Common data types used in ER diagrams:

Numeric: int, integer, bigint, decimal, float, double, numeric
String: string, varchar, char, text, clob
Date/Time: date, time, datetime, timestamp
Boolean: boolean, bool, bit
Other: enum, json, uuid, blob, binary

Complete Example

from mermaid.erdiagram import Entity
 
# User entity
user = Entity("USER")
user.add_attribute("user_id", "int", constraint="PK")
user.add_attribute("username", "string", constraint="UK")
user.add_attribute("email", "string", constraint="UK")
user.add_attribute("password_hash", "string")
user.add_attribute("first_name", "string")
user.add_attribute("last_name", "string")
user.add_attribute("profile_image_url", "string")
user.add_attribute("bio", "text", comment="User biography")
user.add_attribute("created_at", "datetime")
user.add_attribute("updated_at", "datetime")
user.add_attribute("is_active", "boolean")
 
# Post entity
post = Entity("POST")
post.add_attribute("post_id", "int", constraint="PK")
post.add_attribute("user_id", "int", constraint="FK", comment="References USER")
post.add_attribute("title", "string")
post.add_attribute("content", "text")
post.add_attribute("status", "enum", comment="draft, published, archived")
post.add_attribute("published_at", "datetime")
post.add_attribute("view_count", "int")
post.add_attribute("created_at", "datetime")
post.add_attribute("updated_at", "datetime")
 
# Comment entity
comment = Entity("COMMENT")
comment.add_attribute("comment_id", "int", constraint="PK")
comment.add_attribute("post_id", "int", constraint="FK", comment="References POST")
comment.add_attribute("user_id", "int", constraint="FK", comment="References USER")
comment.add_attribute("content", "text")
comment.add_attribute("created_at", "datetime")
comment.add_attribute("updated_at", "datetime")
 
# Tag entity
tag = Entity("TAG")
tag.add_attribute("tag_id", "int", constraint="PK")
tag.add_attribute("name", "string", constraint="UK")
tag.add_attribute("slug", "string", constraint="UK", comment="URL-friendly version")
 
# Junction table for many-to-many relationship
post_tag = Entity("POST_TAG")
post_tag.add_attribute("post_id", "int", constraint="FK PK")
post_tag.add_attribute("tag_id", "int", constraint="FK PK")
post_tag.add_attribute("created_at", "datetime")

Database Design Patterns

Entity for One-to-One Relationship

# User has one profile
user = Entity("USER")
user.add_attribute("user_id", "int", constraint="PK")
# ... other attributes
 
profile = Entity("PROFILE")
profile.add_attribute("profile_id", "int", constraint="PK")
profile.add_attribute("user_id", "int", constraint="FK UK")
profile.add_attribute("bio", "text")
# One-to-one enforced by unique constraint on foreign key

Entity for One-to-Many Relationship

# Department has many employees
department = Entity("DEPARTMENT")
department.add_attribute("dept_id", "int", constraint="PK")
department.add_attribute("name", "string")
 
employee = Entity("EMPLOYEE")
employee.add_attribute("emp_id", "int", constraint="PK")
employee.add_attribute("dept_id", "int", constraint="FK")
employee.add_attribute("name", "string")

Entity for Many-to-Many Relationship

# Students enroll in many courses, courses have many students
student = Entity("STUDENT")
student.add_attribute("student_id", "int", constraint="PK")
student.add_attribute("name", "string")
 
course = Entity("COURSE")
course.add_attribute("course_id", "int", constraint="PK")
course.add_attribute("name", "string")
 
# Junction table
enrollment = Entity("ENROLLMENT")
enrollment.add_attribute("student_id", "int", constraint="FK PK")
enrollment.add_attribute("course_id", "int", constraint="FK PK")
enrollment.add_attribute("grade", "char")
enrollment.add_attribute("enrollment_date", "datetime")

Weak Entity

# Order exists because of Customer; OrderItem existence depends on Order
order_item = Entity("ORDER_ITEM")
order_item.add_attribute("order_id", "int", constraint="FK PK", comment="Composite key")
order_item.add_attribute("item_number", "int", constraint="PK", comment="Composite key")
order_item.add_attribute("product_id", "int", constraint="FK")
order_item.add_attribute("quantity", "int")
order_item.add_attribute("unit_price", "decimal")

Tips

Naming: Use singular nouns for entity names (USER, not USERS)
Attributes: Keep attribute names descriptive and consistent
Keys: Always define primary keys for every entity
Relationships: Use foreign keys to establish relationships
Constraints: Use FK to maintain referential integrity
Uniqueness: Use UK for fields that must be unique (usernames, emails, etc.)
Comments: Document complex attributes and their purposes
Types: Use appropriate data types for clear schema documentation

Graph (Base Class)Erdiagram