# Migration from Sequelize to TypeORM

* [Setting up a connection](#setting-up-a-connection)
* [Schema synchronization](#schema-synchronization)
* [Creating a models](#creating-a-models)
* [Other model settings](#other-model-settings)
* [Working with models](#working-with-models)

## Setting up a connection

In sequelize you create a connection this way:

```javascript
const sequelize = new Sequelize("database", "username", "password", {
  host: "localhost",
  dialect: "mysql"
});

sequelize
  .authenticate()
  .then(() => {
    console.log("Connection has been established successfully.");
  })
  .catch(err => {
    console.error("Unable to connect to the database:", err);
  });
```

In TypeORM you create a connection like this:

```typescript
import {createConnection} from "typeorm";

createConnection({
    type: "mysql",
    host: "localhost",
    username: "username",
    password: "password"
}).then(connection => {
    console.log("Connection has been established successfully.");
})
.catch(err => {
    console.error("Unable to connect to the database:", err);
});
```

Then you can get your connection instance from anywhere in your app using `getConnection`.

## Schema synchronization

In sequelize you do schema synchronization this way:

```javascript
Project.sync({force: true});
Task.sync({force: true});
```

In TypeORM you just add `synchronize: true` in the connection options:

```typescript
createConnection({
    type: "mysql",
    host: "localhost",
    username: "username",
    password: "password",
    synchronize: true
});
```

## Creating a models

This is how models are defined in sequelize:

```javascript
module.exports = function(sequelize, DataTypes) {

    const Project = sequelize.define("project", {
      title: DataTypes.STRING,
      description: DataTypes.TEXT
    });
    
    return Project;

};
```

```javascript
module.exports = function(sequelize, DataTypes) {

    const Task = sequelize.define("task", {
      title: DataTypes.STRING,
      description: DataTypes.TEXT,
      deadline: DataTypes.DATE
    });
    
    return Task;
};
```

In TypeORM these models are called entities and you can define them like this:

```typescript
import {Entity, PrimaryGeneratedColumn, Column} from "typeorm";

@Entity()
export class Project {
    
    @PrimaryGeneratedColumn()
    id: number;
    
    @Column()
    title: string;
    
    @Column()
    description: string;
    
}
```

```typescript
import {Entity, PrimaryGeneratedColumn, Column} from "typeorm";

@Entity()
export class Task {
    
    @PrimaryGeneratedColumn()
    id: number;
    
    @Column()
    title: string;
    
    @Column("text")
    description: string;
    
    @Column()
    deadline: Date;
    
}
```

It's highly recommended to define one entity class per file.
TypeORM allows you to use your classes as database models
and provides a declarative way to define what part of your model 
will become part of your database table.
The power of TypeScript gives you type hinting and other useful features that you can use in classes.

## Other model settings

The following in sequelize:

```javascript
flag: { type: Sequelize.BOOLEAN, allowNull: true, defaultValue: true },
```

Can be achieved in TypeORM like this:

```typescript
@Column({ nullable: true, default: true })
flag: boolean;
```

Following in sequelize:

```javascript
flag: { type: Sequelize.DATE, defaultValue: Sequelize.NOW }
```

Is written like this in TypeORM:

```typescript
@Column({ default: () => "NOW()" })
myDate: Date;
```

Following in sequelize:

```javascript
someUnique: { type: Sequelize.STRING, unique: true },
```

Can be achieved this way in TypeORM:

```typescript
@Column({ unique: true })
someUnique: string;
```

Following in sequelize:

```javascript
fieldWithUnderscores: { type: Sequelize.STRING, field: "field_with_underscores" },
```

Translates to this in TypeORM:

```typescript
@Column({ name: "field_with_underscores" })
fieldWithUnderscores: string;
```

Following in sequelize:

```javascript
incrementMe: { type: Sequelize.INTEGER, autoIncrement: true },
```

Can be achieved this way in TypeORM:

```typescript
@Column()
@Generated()
incrementMe: number;
```

Following in sequelize:

```javascript
identifier: { type: Sequelize.STRING, primaryKey: true },
```

Can be achieved this way in TypeORM:

```typescript
@Column({ primary: true })
identifier: string;
```

To create `createDate` and `updateDate`-like columns you need to defined two columns (name it what you want) in your entity:

```typescript
@CreateDateColumn();
createDate: Date;

@UpdateDateColumn();
updateDate: Date;
``` 

### Working with models

To create a new model in sequelize you write:

```javascript
const employee = await Employee.create({ name: "John Doe", title: "senior engineer" });
```

In TypeORM there are several ways to create a new model:

```typescript
const employee = new Employee(); // you can use constructor parameters as well
employee.name = "John Doe";
employee.title = "senior engineer";
```

or 

```typescript
const employee = Employee.create({ name: "John Doe", title: "senior engineer" });
```

if you want to load an existing entity from the database and replace some of its properties you can use the following method:

```typescript
const employee = await Employee.preload({ id: 1, name: "John Doe" });
```

To access properties in sequelize you do the following:

```typescript
console.log(employee.get("name"));
```

In TypeORM you simply do:

```typescript
console.log(employee.name);
```

To create an index in sequelize you do:

```typescript
sequelize.define("user", {}, {
  indexes: [
    {
      unique: true,
      fields: ["firstName", "lastName"]
    }
  ]
});
```

In TypeORM you do:

```typescript
@Entity()
@Index(["firstName", "lastName"], { unique: true })
export class User {
}
```