Introduce mermaid entity relationship diagram

README.md

@@ -9,7 +9,7 @@
 # What is markdown package
 The Package markdown is a simple markdown builder in golang. The markdown package assembles Markdown using method chaining, not uses a template engine like [html/template](https://pkg.go.dev/html/template). The syntax of Markdown follows **GitHub Markdown**.
 
-The markdown package was initially developed to save test results in [nao1215/spectest](https://github.com/nao1215/spectest). Therefore, the markdown package implements the features required by spectest. For example, the markdown package supports **mermaid sequence diagrams**, which was a necessary feature in spectest.
+The markdown package was initially developed to save test results in [nao1215/spectest](https://github.com/nao1215/spectest). Therefore, the markdown package implements the features required by spectest. For example, the markdown package supports **mermaid sequence diagrams (entity relationship diagram, sequence diagram, pie chart)**, which was a necessary feature in spectest.
 
 Additionally, complex code that increases the complexity of the library, such as generating nested lists, will not be added. I want to keep this library as simple as possible.
 
@@ -33,6 +33,7 @@ Additionally, complex code that increases the complexity of the library, such as
 - [x] Details 
 - [x] Alerts; NOTE, TIP, IMPORTANT, CAUTION, WARNING
 - [x] mermaid sequence diagram
+- [x] mermaid entity relationship diagram
 - [x] mermaid pie chart
 
 ### Features not in Markdown syntax
@@ -411,6 +412,182 @@ pie showData
     "C" : 30
 ```
 
+### Entity Relationship Diagram syntax
+
+```go
+package main
+
+import (
+	"os"
+
+	"github.com/nao1215/markdown"
+	"github.com/nao1215/markdown/mermaid/er"
+)
+
+//go:generate go run main.go
+
+func main() {
+	f, err := os.Create("generated.md")
+	if err != nil {
+		panic(err)
+	}
+
+	teachers := er.NewEntity(
+		"teachers",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "Teacher Name",
+			},
+		},
+	)
+	students := er.NewEntity(
+		"students",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "Student ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "Student Name",
+			},
+			{
+				Type:         "int",
+				Name:         "teacher_id",
+				IsPrimaryKey: false,
+				IsForeignKey: true,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+		},
+	)
+	schools := er.NewEntity(
+		"schools",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "School ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "School Name",
+			},
+			{
+				Type:         "int",
+				Name:         "teacher_id",
+				IsPrimaryKey: false,
+				IsForeignKey: true,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+		},
+	)
+
+	erString := er.NewDiagram(f).
+		Relationship(
+			teachers,
+			students,
+			er.ExactlyOneRelationship, // "||"
+			er.ZeroToMoreRelationship, // "}o"
+			er.Identifying,            // "--"
+			"Teacher has many students",
+		).
+		Relationship(
+			teachers,
+			schools,
+			er.OneToMoreRelationship,  // "|}"
+			er.ExactlyOneRelationship, // "||"
+			er.NonIdentifying,         // ".."
+			"School has many teachers",
+		).
+		String()
+
+	err = markdown.NewMarkdown(f).
+		H2("Entity Relationship Diagram").
+		CodeBlocks(markdown.SyntaxHighlightMermaid, erString).
+		Build()
+
+	if err != nil {
+		panic(err)
+	}
+}
+```
+
+Plain text output: [markdown is here](./doc/er/generated.md)
+````
+## Entity Relationship Diagram
+```mermaid
+erDiagram
+	teachers ||--o{ students : "Teacher has many students"
+	teachers }|..|| schools : "School has many teachers"
+	schools {
+		int id PK,UK "School ID"
+		string name  "School Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	students {
+		int id PK,UK "Student ID"
+		string name  "Student Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	teachers {
+		int id PK,UK "Teacher ID"
+		string name  "Teacher Name"
+	}
+
+```
+````
+
+Mermaid output:
+```mermaid
+erDiagram
+	teachers ||--o{ students : "Teacher has many students"
+	teachers }|..|| schools : "School has many teachers"
+	schools {
+		int id PK,UK "School ID"
+		string name  "School Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	students {
+		int id PK,UK "Student ID"
+		string name  "Student Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	teachers {
+		int id PK,UK "Teacher ID"
+		string name  "Teacher Name"
+	}
+```
+
 ## Creating an index for a directory full of markdown files
 The markdown package can create an index for Markdown files within the specified directory. This feature was added to generate indexes for Markdown documents produced by [nao1215/spectest](https://github.com/nao1215/spectest).
 

doc/er/generated.md

@@ -0,0 +1,21 @@
+## Entity Relationship Diagram
+```mermaid
+erDiagram
+	teachers ||--o{ students : "Teacher has many students"
+	teachers }|..|| schools : "School has many teachers"
+	schools {
+		int id PK,UK "School ID"
+		string name  "School Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	students {
+		int id PK,UK "Student ID"
+		string name  "Student Name"
+		int teacher_id FK,UK "Teacher ID"
+	}
+	teachers {
+		int id PK,UK "Teacher ID"
+		string name  "Teacher Name"
+	}
+
+```

doc/er/main.go

@@ -0,0 +1,128 @@
+//go:build linux || darwin
+
+// Package main is generating  entity relationship diagram.
+package main
+
+import (
+	"os"
+
+	"github.com/nao1215/markdown"
+	"github.com/nao1215/markdown/mermaid/er"
+)
+
+//go:generate go run main.go
+
+func main() {
+	f, err := os.Create("generated.md")
+	if err != nil {
+		panic(err)
+	}
+
+	teachers := er.NewEntity(
+		"teachers",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "Teacher Name",
+			},
+		},
+	)
+	students := er.NewEntity(
+		"students",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "Student ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "Student Name",
+			},
+			{
+				Type:         "int",
+				Name:         "teacher_id",
+				IsPrimaryKey: false,
+				IsForeignKey: true,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+		},
+	)
+	schools := er.NewEntity(
+		"schools",
+		[]*er.Attribute{
+			{
+				Type:         "int",
+				Name:         "id",
+				IsPrimaryKey: true,
+				IsForeignKey: false,
+				IsUniqueKey:  true,
+				Comment:      "School ID",
+			},
+			{
+				Type:         "string",
+				Name:         "name",
+				IsPrimaryKey: false,
+				IsForeignKey: false,
+				IsUniqueKey:  false,
+				Comment:      "School Name",
+			},
+			{
+				Type:         "int",
+				Name:         "teacher_id",
+				IsPrimaryKey: false,
+				IsForeignKey: true,
+				IsUniqueKey:  true,
+				Comment:      "Teacher ID",
+			},
+		},
+	)
+
+	erString := er.NewDiagram(f).
+		Relationship(
+			teachers,
+			students,
+			er.ExactlyOneRelationship, // "||"
+			er.ZeroToMoreRelationship, // "}o"
+			er.Identifying,            // "--"
+			"Teacher has many students",
+		).
+		Relationship(
+			teachers,
+			schools,
+			er.OneToMoreRelationship,  // "|}"
+			er.ExactlyOneRelationship, // "||"
+			er.NonIdentifying,         // ".."
+			"School has many teachers",
+		).
+		String()
+
+	err = markdown.NewMarkdown(f).
+		H2("Entity Relationship Diagram").
+		CodeBlocks(markdown.SyntaxHighlightMermaid, erString).
+		Build()
+
+	if err != nil {
+		panic(err)
+	}
+}

doc/piechart/main.go

@@ -1,6 +1,6 @@
 //go:build linux || darwin
 
-// Package main is generating mermaid sequence diagram.
+// Package main is generating pie chart.
 package main
 
 import (

mermaid/er/config.go

@@ -0,0 +1,12 @@
+package er
+
+// config is the configuration for the entity relationship diagram.
+type config struct{}
+
+// newConfig returns a new config.
+func newConfig() *config {
+	return &config{}
+}
+
+// Option sets the options for the PieChart struct.
+type Option func(*config)