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   *  regarding copyright ownership.  The ASF licenses this file
   *  to you under the Apache License, Version 2.0 (the
   *  "License"); you may not use this file except in compliance
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   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.map;
  
  import java.io.Serializable;
  
  import org.apache.cayenne.CayenneRuntimeException;
  import org.apache.cayenne.configuration.ConfigurationNodeVisitor;
  import org.apache.cayenne.util.ToStringBuilder;
  import org.apache.cayenne.util.XMLEncoder;
  import org.apache.cayenne.util.XMLSerializable;
  
  /**
   * Defines a join between two attributes of a given relationship.
   * 
   * @since 1.1
   */
  public class DbJoin implements XMLSerializable, Serializable {
  
      protected DbRelationship relationship;
      protected String sourceName;
      protected String targetName;
  
      protected DbJoin() {
      }
  
      public DbJoin(DbRelationship relationship) {
          this.relationship = relationship;
      }
  
      public DbJoin(DbRelationship relationship, String sourceName, String targetName) {
          this.relationship = relationship;
          this.sourceName = sourceName;
          this.targetName = targetName;
      }
  
      /**
       * Returns a "reverse" join. Join source relationship is not set and must be
       * initialized by the caller.
       */
      public DbJoin createReverseJoin() {
          DbJoin reverse = new DbJoin();
          reverse.setTargetName(sourceName);
          reverse.setSourceName(targetName);
          return reverse;
      }
  
      /**
       * Returns DbAttribute on on the left side of the join.
       */
      public DbAttribute getSource() {
          if (sourceName == null) {
              return null;
          }
  
          DbRelationship r = getNonNullRelationship();
          DbEntity entity = r.getSourceEntity();
          if (entity == null) {
              return null;
          }
  
          return entity.getAttribute(sourceName);
      }
  
      public DbAttribute getTarget() {
          if (targetName == null) {
              return null;
          }
  
          DbRelationship r = getNonNullRelationship();
          DbEntity entity = r.getTargetEntity();
          if (entity == null) {
              return null;
          }
  
          return entity.getAttribute(targetName);
      }
  
      /**
       * Prints itself as XML to the provided XMLEncoder.
       */
      @Override
      public void encodeAsXML(XMLEncoder encoder, ConfigurationNodeVisitor delegate) {
          encoder.start("db-attribute-pair")
                  .attribute("source", getSourceName())
                  .attribute("target", getTargetName())
                  .end();
      }
  
      public DbRelationship getRelationship() {
          return relationship;
      }
  
      public String getSourceName() {
          return sourceName;
      }
  
      public String getTargetName() {
          return targetName;
      }
  
      public void setRelationship(DbRelationship relationship) {
          this.relationship = relationship;
      }
  
      public void setSourceName(String string) {
          sourceName = string;
      }
  
      public void setTargetName(String string) {
          targetName = string;
      }
  
      private DbRelationship getNonNullRelationship() {
          if (relationship == null) {
              throw new CayenneRuntimeException("Join has no parent Relationship.");
          }
  
          return relationship;
      }
  
      @Override
      public String toString() {
          ToStringBuilder builder = new ToStringBuilder(this);
          builder.append("source", getSourceName());
          builder.append("target", getTargetName());
          return builder.toString();
      }
  }
  