Add Relationship Between Entities

This example will show you how you can add relationships between entities similar to database.

Java code listing

package com.northconcepts.datapipeline.foundations.examples.schema;

import com.northconcepts.datapipeline.core.FieldType;
import com.northconcepts.datapipeline.foundations.schema.EntityDef;
import com.northconcepts.datapipeline.foundations.schema.EntityRelationshipDef;
import com.northconcepts.datapipeline.foundations.schema.ForeignKeyAction;
import com.northconcepts.datapipeline.foundations.schema.NumericFieldDef;
import com.northconcepts.datapipeline.foundations.schema.PrimaryKeyType;
import com.northconcepts.datapipeline.foundations.schema.RelationshipCardinality;
import com.northconcepts.datapipeline.foundations.schema.SchemaDef;
import com.northconcepts.datapipeline.foundations.schema.TextFieldDef;

public class AddRelationshipBetweenEntities {

    public static void main(String[] args) {
        SchemaDef schema = new SchemaDef("E-Commerce Schema");

        schema.addEntity(new EntityDef("orders")
                .addField(new NumericFieldDef("order_id", FieldType.LONG).setPrimaryKeyPosition(0).setPrimaryKeyType(PrimaryKeyType.TABLE))
                .addField(new NumericFieldDef("customer_id", FieldType.LONG))
                .addField(new TextFieldDef("product_name", FieldType.STRING))
                .addField(new NumericFieldDef("quantity", FieldType.LONG))
                );

        schema.addEntity(new EntityDef("customers")
                .addField(new NumericFieldDef("customer_id", FieldType.LONG).setPrimaryKeyPosition(0).setPrimaryKeyType(PrimaryKeyType.AUTO))
                .addField(new TextFieldDef("customer_name", FieldType.STRING))
                .addField(new TextFieldDef("address", FieldType.STRING))
                );

        EntityRelationshipDef entityRelationship = new EntityRelationshipDef("fk_customers_orders")
                .setPrimaryEntityName("customers")
                .setForeignEntityName("orders")
                .setCardinality(RelationshipCardinality.ONE_TO_MANY)
                .setForeignKeyFieldNames("customer_id")
                .setOnUpdateAction(ForeignKeyAction.SET_DEFAULT)
                .setOnDeleteAction(ForeignKeyAction.SET_NULL);
        schema.addEntityRelationship(entityRelationship);
        
        System.out.println("=========================================================");
        System.out.println(schema.toXml());
        System.out.println("=========================================================");
    }
}

Code walkthrough

1. Create a SchemaDef named E-Commerce Schema with two entities.
2. An entity, customers has one primary key - customer_id.
3. Another entity, orders has a field - customer_id which is foreign key and referred from customers entity.
4. Create EntityRelationshipDef to define the relationship between two entities.
5. Here customers is primary entity and orders is foreign entity.
6. You can also define properties similar to SQL database e.g. RelationshipCardinality, ForeignKeyAction
7. Print this schema as XML.

Console Output

=========================================================
<schema name="E-Commerce Schema">
  <entities>
    <entity addMissingOptionalFields="false" allowExtraFieldsInMapping="true" allowExtraFieldsInValidation="true" name="orders">
      <fields>
        <field array="false" name="order_id" primaryKeyPosition="0" primaryKeyType="TABLE" required="false" strictArrays="true" type="LONG"/>
        <field array="false" name="customer_id" required="false" strictArrays="true" type="LONG"/>
        <field allowBlank="true" array="false" name="product_name" required="false" strictArrays="true" type="STRING"/>
        <field array="false" name="quantity" required="false" strictArrays="true" type="LONG"/>
      </fields>
    </entity>
    <entity addMissingOptionalFields="false" allowExtraFieldsInMapping="true" allowExtraFieldsInValidation="true" name="customers">
      <fields>
        <field array="false" name="customer_id" primaryKeyPosition="0" primaryKeyType="AUTO" required="false" strictArrays="true" type="LONG"/>
        <field allowBlank="true" array="false" name="customer_name" required="false" strictArrays="true" type="STRING"/>
        <field allowBlank="true" array="false" name="address" required="false" strictArrays="true" type="STRING"/>
      </fields>
    </entity>
  </entities>
  <entity-relationships>
    <entity-relationship cardinality="ONE_TO_MANY" foreignEntityName="orders" name="fk_customers_orders" onDeleteAction="SET_NULL" onUpdateAction="SET_DEFAULT" primaryEntityName="customers">
      <foreign-key name="customer_id"/>
    </entity-relationship>
  </entity-relationships>
</schema>

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