Enum and Derived Attributes

Creating enums using the platform

The EntryType class identified during the design phase is an enumeration. The JSON blueprint for the same, is supposed to look as follows :

{
  "entry_type": {
    "purpose": "POSSIBLE_VALUE",
    "values": {
      ...
    }
  }
}

Notice that instead of using RESOURCE as the purpose, we specify it as POSSIBLE_VALUE to denote an enum. In the values field, we are expected to specify the different constants that define the set of elements of the enum. Each constant has an id field and a name field. This is illustrated in the example below :

{
  "entry_type": {
    "purpose": "POSSIBLE_VALUE",
    "values": {
      "going_out": {
        "id": "going_out",
        "name": "Going Out"
      },
      "coming_in": {
        "id": "coming_in",
        "name": "Coming In"
      },
      "going_home": {
        "id": "going_home",
        "name": "Going Home"
      },
      "coming_from_home": {
        "id": "coming_from_home",
        "name": "Coming From Home"
      }
    }
  }
}

---

Enums as data types

Once we have the JSON specification for the EntryType enum ready, we can use it in the StudentMovement class as the data type for the entry_type field.

This is done by mentioning the type field of the entry_type attribute as String and then using the possible_value field to specify the corresponding enum class along with the correct package descriptor.

"entry_type": {
	"length": 128,
	"type": "String",
	"possible_value": "{groupId}.{artifactId}.resource.EntryType"
}

Note that the class name EntryType is qualified by the correct name of the package it is supposed to belong to once the resources that been generated. This can be verified by creating a Java class in the src/main/java/{groupId}/{artifactId}/resource directory using an IDE like IntelliJ IDEA.

---

Foreign Derived Attributes

Consider the student_batch_name attribute of the Student class. The singleton set {student_batch_name} is functionally dependent on the singleton set {student_batch_id}. In other words, if the value of student_batch_id is known for a particular student, the value of student_batch_name is uniquely determined and can be fetched from the Batch table. This makes the latter a derived attribute.

The platform is equipped to handle such situations without requiring custom logic to be written. We can address this particular case as follows :

1. Since student_batch_id shall behave as the foreign key to the Batch table, we add the foreign attribute to its JSON blueprint :

"student_batch_id": {
	"index": true,
	"name": "student_batch_id",
	"length": 128,
	"type": "String",

	"foreign": {
	  "resource": "batch"
	}
}

The resource attribute points to the Batch table referenced by the foreign key student_batch_id.

2. Now, to ensure that the values of the student_batch_name field are filled in using values from the Batch table, we add the foreign_derived attribute to its blueprint.

"student_batch_name": {
	"length": 128,
	"type": "String",
	
	"foreign_derived": {
	  "parent_field": "student_batch_id",
	  "foreign_field": "name"
	}
}

• parent_field : Denotes the foreign key in the Student table.
• foreign_field : Denotes the attribute in the Batch table from which the value of student_batch_name is derived from.

Now, when one creates a student, only the student_batch_id field needs to be supplied. And while querying for students, the student_batch_name field will be filled in from the Batch table.

This puts us in a position to completely specify the JSON blueprint for the StudentMovement class - complete with derived fields and enums :

{
  "student_movement": {
    "cluster": "DB_PROFILE",
    "ui": false,
    "api": true,
    "display_name": "Student Movement",
    "fields": {
      "id": {
        "index": true,
        "name": "id",
        "length": 16,
        "required": true,
        "type": "String"
      },
      "student_id": {
        "name": "student_id", "length": 128, "type": "String",
        "foreign": {
          "resource": "student"
        }
      },
      "student_name": {
        "length": 128, "type": "String", "name": "student_name",
        "foreign_derived": {
          "parent_field": "student_id",
          "foreign_field": "student_name"
        }
      },
      "entry_time": {
        "type": "timestamp", "name": "entry_time",
      },
      "entry_gate": {
        "type": "String", "length": 128, "name": "entry_gate",
      },
      "home_entry": {
        "type": "boolean", "name": "home_entry",
      },
      "entry_type": {
        "length": 128, "type": "String",
        "possible_value": "org.rasp.iiitb360.resource.EntryType"
      },
      "student_batch_id": {
        "name": "student_batch_id", "length": 128, "type": "String",
        "foreign_derived": {
          "parent_field": "student_id",
          "foreign_field": "student_batch_id"
        }
      }
    }
  }
}