ER Diagram

ER diagrams show database entities, their columns, and the relationships between tables. Use them to document schema designs or plan data models.

Minimal Example

{
  "type": "er",
  "entities": [
    { "id": "user", "name": "User", "columns": [{ "name": "id", "type": "uuid", "pk": true }] },
    { "id": "post", "name": "Post", "columns": [{ "name": "id", "type": "uuid", "pk": true }] }
  ],
  "relationships": [
    { "from": "user", "to": "post", "cardinality": "1:N" }
  ]
}

Full Example

{
  "type": "er",
  "entities": [
    {
      "id": "user",
      "name": "User",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "email", "type": "varchar", "unique": true },
        { "name": "name", "type": "varchar" },
        { "name": "created_at", "type": "timestamp" }
      ]
    },
    {
      "id": "order",
      "name": "Order",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "user_id", "type": "uuid", "fk": true },
        { "name": "total", "type": "decimal" },
        { "name": "status", "type": "varchar" }
      ]
    },
    {
      "id": "product",
      "name": "Product",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "name", "type": "varchar" },
        { "name": "price", "type": "decimal" }
      ]
    },
    {
      "id": "orderItem",
      "name": "OrderItem",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "order_id", "type": "uuid", "fk": true },
        { "name": "product_id", "type": "uuid", "fk": true },
        { "name": "quantity", "type": "integer" }
      ]
    }
  ],
  "relationships": [
    { "from": "user", "to": "order", "cardinality": "1:N", "label": "places" },
    { "from": "order", "to": "orderItem", "cardinality": "1:N", "label": "contains" },
    { "from": "product", "to": "orderItem", "cardinality": "1:N", "label": "appears in" }
  ]
}

Entities

Each entity represents a database table.

Field	Required	Description
`id`	Yes	Unique identifier — relationships reference this
`name`	Yes	Display name shown in the entity header
`columns`	Yes	Array of column objects

Columns

Each column describes a table field.

Field	Required	Description
`name`	Yes	Column name
`type`	Yes	Data type (e.g., `"uuid"`, `"varchar"`, `"integer"`)
`pk`	No	`true` to mark as primary key
`fk`	No	`true` to mark as foreign key
`unique`	No	`true` to mark as unique

{ "name": "email", "type": "varchar", "unique": true }

Relationships

Each relationship connects two entities.

Field	Required	Description
`from`	Yes	Source entity `id`
`to`	Yes	Target entity `id`
`cardinality`	Yes	`"1:1"`, `"1:N"`, `"N:1"`, or `"M:N"`
`label`	No	Description of the relationship

Cardinality

Value	Meaning
`"1:1"`	One-to-one
`"1:N"`	One-to-many
`"N:1"`	Many-to-one
`"M:N"`	Many-to-many

More Examples

Blog Platform

{
  "type": "er",
  "entities": [
    {
      "id": "author",
      "name": "Author",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "username", "type": "varchar", "unique": true },
        { "name": "bio", "type": "text" }
      ]
    },
    {
      "id": "post",
      "name": "Post",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "author_id", "type": "uuid", "fk": true },
        { "name": "title", "type": "varchar" },
        { "name": "body", "type": "text" },
        { "name": "published_at", "type": "timestamp" }
      ]
    },
    {
      "id": "tag",
      "name": "Tag",
      "columns": [
        { "name": "id", "type": "uuid", "pk": true },
        { "name": "name", "type": "varchar", "unique": true }
      ]
    },
    {
      "id": "postTag",
      "name": "PostTag",
      "columns": [
        { "name": "post_id", "type": "uuid", "fk": true },
        { "name": "tag_id", "type": "uuid", "fk": true }
      ]
    }
  ],
  "relationships": [
    { "from": "author", "to": "post", "cardinality": "1:N", "label": "writes" },
    { "from": "post", "to": "postTag", "cardinality": "1:N" },
    { "from": "tag", "to": "postTag", "cardinality": "1:N" }
  ]
}

On this page