Diagrams: ER and Schema¶

This page covers ERDiagram (Chen notation -professional- style) and SchemaDiagram (plain database table with FK lines).

ERDiagram¶

Entity-Relationship diagrams use Chen notation: rectangles for entities, diamonds for relationships, rounded rectangles or ovals for attributes.

Quick example¶

import engrapha_notes as en
import engrapha_diagrams as ed

er = ed.ERDiagram(width=450, height=220, caption="Fig 3: University ER Model")

er.entity("STUDENT")
er.entity("COURSE")
er.relationship("ENROLLS")

er.entity_attributes(
    "STUDENT",
    [("Student_ID", {"pk": True}), "Name", ("Email", {"derived": True})],
)
er.entity_attributes(
    "COURSE",
    [("Course_ID", {"pk": True}), "Title"],
)

er.connect("STUDENT", "ENROLLS", card_from="1", card_to="N")
er.connect("COURSE", "ENROLLS", card_from="1", card_to="N")

en.add(er.as_flowable())
en.build_doc("er.pdf")

Entities¶

er.entity("STUDENT", weak=False)   # Strong (default)
er.entity("COURSE", weak=True)     # Weak

Relationships¶

er.relationship("ENROLLS", identifying=True)

Attributes¶

er.attribute("Grade", parent="ENROLLS", x=225, y=55)

Or use entity_attributes(...) for concentric-circle layout:

er.entity_attributes(
    "STUDENT",
    [
        ("ID",          {"pk": True}),
        "Name",
        ("Email",       {"derived": True}),
        ("Phone",       {"multivalued": True}),
    ],
    distance=70.0,
    start_angle=90.0,
)

Connections¶

er.connect("STUDENT", "ENROLLS",
           card_from="1", card_to="N",
           total_from=True, total_to=False)

Boundary conditions:

total_from / total_to uses a double-line symbol (total participation).
Several cardinality label strings supported: "1", "N", "M", "0..1".

SchemaDiagram¶

Schema diagrams display table structures: columns, types, primary and foreign key markers, plus labelled relationship arcs.

Quick example¶

import engrapha_notes as en
import engrapha_diagrams as ed

schema = ed.SchemaDiagram(
    width=450, height=200, caption="Fig 4: Blog Schema"
)

schema.table("users", [
    ("id",         "INTEGER", {"pk": True}),
    ("email",      "VARCHAR", {}),
    ("created_at", "TIMESTAMP", {}),
])

schema.table("posts", [
    ("id",          "INTEGER",  {"pk": True}),
    ("user_id",     "INTEGER",  {"fk": True}),
    ("title",       "VARCHAR",  {}),
    ("published_at", "TIMESTAMP", {}),
])

schema.relation("posts", "user_id", "users", "id")

en.add(schema.as_flowable())
en.build_doc("schema.pdf")

Column flags¶

schema.table("orders", [
    ("id",        "INTEGER", {"pk": True}),    # Primary Key
    ("user_id",   "INTEGER", {"fk": True}),     # Foreign Key
    ("total",     "DECIMAL", {}),
])

pk=True: PK appended, bold king-colored text
fk=True: (FK) appended, relation-colored text

Auto layout¶

If every table is at (0, 0) (the default), SchemaDiagram distributes them on a grid automatically:

schema.table("users", [...])
schema.table("orders", [...])
schema.table("products", [...])
# → auto-positioned in a 2×2 grid

Choosing between ER and Schema¶

Use ERDiagram when you care about:

Chen notation (entity rectangles or rounded, double diamonds)
Weak vs strong entities
Cardinalities (1, N, M)
Identifying relationships

Use SchemaDiagram when you care about:

Table-like view with column types
Primary / Foreign key markers
Quick SQL-style wireframes

Next¶

Schema
Class