Derived entity types

Ballerina record types used in the generated clients are derived from the entity types defined in the data model.

Below is an example of a defined entity.

type Workspace record {|
    readonly string id;
    string workspaceType;
    Building location; // 1-n relation (parent)
    Employee? employees; // 1-1 relation (child)
|};

There are six derived entity types as follows

Entity types

These are the types defined in the data model. They are used to indicate the data structure in the data source. The entity, which is the association parent will include the foreign key field(s).
```ballerina
public type Workspace record {|
    readonly string id;
    string workspaceType;
    string locationCode;
|};
```

Insert types

These are the records used to insert data in the data source. This is the same as the entity type.
```ballerina
public type WorkspaceInsert Workspace;
```

Update types

These are the records used to update data in the data source. They are entity types without identity fields. All fields will be optional. Only the fields for which values are provided will be updated.
```ballerina
public type WorkspaceUpdate record {|
    string workspaceType?;
    string locationCode?;
|};
```

Optionalized types

These are the same as the entity types but all the fields are made optional. This type is not directly used in any operations.
```ballerina
public type WorkspaceOptionalized record {|
    readonly string id?;
    string workspaceType?;
    string locationCode?;
|};
```

With-relations types

These types inherit all fields from the corresponding `optionalized` type and add the relation fields as optional. They are not directly used in any operations.
```ballerina
public type WorkspaceWithRelations record {|
    *WorkspaceOptionalized;
    BuildingOptionalized location?;
    EmployeeOptionalized employees?;
|};
```

Target types

These types are used to retrieve data from the data source. If the entity contains a relation field, the target type 
will be a type description of the corresponding `WithRelations` type. Otherwise, the target type will be a 
type description of the corresponding `optionalized` type.
```ballerina
public type WorkspaceTargetType typedesc<WorkspaceWithRelations>;
```

Client Object

The client object is derived for each data model definition file, and it is used to perform CRUD operations on the data source. Each entity type will have five resource methods for each operation. The resource methods are get, get(get by identity), post, put, and delete.

The skeleton of the client object is as follows.

public client class Client {
    *persist:AbstractPersistClient;

    public function init() returns persist:Error? {
    }

    isolated resource function get workspaces(WorkspaceTargetType targetType = <>) returns stream<targetType, Error?> {
    };

    isolated resource function get workspaces/[string id](WorkspaceTargetType targetType = <>) returns targetType|Error {
    };

    isolated resource function post workspaces(WorkspaceInsert[] data) returns string[]|persist:Error {
    };

    isolated resource function put workspaces/[string id](WorkspaceUpdate data) returns Workspace|persist:Error {
    };

    isolated resource function delete workspaces/[string id]() returns Workspace|persist:Error {
    };

    public function close() returns persist:Error? {
    }
}

The conventions used in deriving the client object is as follows.

1. Since there can be only one generated client in a Ballerina package, the client name is always Client.
2. The client should be of the persist:AbstractPersistClient type.
3. It should contain the init() and close() functions.
4. It should contain five resource methods (i.e., get, get(get by identity), post, put, and delete for each entity type defined in the data model.
5. Resource names should be in the plural form of the entity names in lowercase.
6. The resource method should return the derived entity types.
7. Resource methods with path parameters will support composite identity fields by having multiple path parameters.

Client usage

Client initialization

The init() function is used to initialize the client. The client will be initialized with the data source configuration provided in the Config.toml file. The init() function will return an error if the client initialization fails.

You can initialize the client as follows.

Client sClient = check new();

CRUD operations

you can perform CRUD operations on the Workspace table in the data store using the client object as follows.

// Create a new workspace record.
WorkspaceInsert workspace = {id: "WK001", workspaceType: "Host Desk", locationCode: "B001"};
string[]|error workspaceId = sClient->/workspaces.post([workspace]);

// Get the workspace record with the ID: `WK001`.
Workspace|error workspace = sClient->/workspaces/WK001;

// Update the workspace record with the ID: `WK001`.
Workspace|error updated = sClient->/workspaces/WK001.put({workspaceType: "Cubical"});

// Delete the workspace record with the ID: `WK001`.
Workspace|error deleted = sClient->/workspaces/WK001.delete();

// Get all the workspace records.
stream<Workspace, error?> workspaces = sClient->/workspaces;

Relation operations

You can perform relation operations on the Workspace table in the data store using the client object as follows.

Let's extend the Workspace entity type with a relation field employees as follows.

type Workspace record {|
    readonly string id;
    string workspaceType;
    Building location; // 1-n relation (parent)
    Employee? employees; // 1-1 relation (child)
|};

type Employee record {|
    readonly string id;
    string firstName;
    string lastName;
    string email;
    float salary;
    Workspace workspace; // 1-1 relation (parent)
|};

type Building record {|
    readonly string code;
    string name;
    string address;
    Workspace[] workspaces; // 1-n relation (child)
|};

You can retrieve Workspace records with the related Employee record by using a custom target type as follows. This is equivalent to an SQL join operation between the Workspace and Employee tables.

// Get the workspace record with the ID `WK001` with the related employee record.
WorkspaceWithEmployee|error workspace = sClient->/workspaces/WK001.get();

// `WorkspaceWithEmployee` is a record with the following fields.
type WorkspaceWithEmployee record {|
    readonly string id;
    string workspaceType;
    string locationCode
    Employee employees; // 1-1 relation (child)
|};

This is the same as writing the following SQL query to retrieve data from a relational database,

SELECT * 
FROM workspace w
INNER JOIN employee e ON w.id = e.workspaceId
WHERE w.workspaceId = 'WK001';

The persisted client will automatically perform the join operation and return the result.

Projection operations

You can retrieve the Workspace records with the related Employee record and the related Building record by using a custom target type. When specifying the target type, you can specify the target type with a subset of fields of the entity types as follows.

This is equivalent to an SQL join operation between the Workspace, Employee, and Building tables, and also an SQL projection operation.

// Get the `workspace` record with the ID `WK001` with the related `employee` record and the related `building` record.

WorkspaceWithRelations|error workspace = sClient->/workspaces/WK001.get();

// `WorkspaceWithRelations` is a record with the following fields.
type WorkspaceWithRelations record {|
    string workspaceType;
    record {|
        string firstName;
        string lastName;
    |} employees; // 1-1 relation (child)
    record {|
        string name;
        string address;
    |} location; // 1-n relation (parent)
|};

This is the same as writing the following SQL query to retrieve data from a relational database.

SELECT w.workspaceType, e.firstName, e.lastName, b.name, b.address
FROM workspace w
INNER JOIN employee e ON w.id = e.workspaceId
INNER JOIN building b ON w.locationCode = b.code
WHERE w.workspaceId = 'WK001';

The client will automatically perform the join and projection operations on the data source and return the result.

Learn more

For more information on the client API usage, see the bal persist examples.