diff --git a/src/ByPropertyIdMap.php b/src/ByPropertyIdMap.php
new file mode 100644
index 00000000..8eecbfd1
--- /dev/null
+++ b/src/ByPropertyIdMap.php
@@ -0,0 +1,154 @@
+<?php
+
+namespace Wikibase\DataModel;
+
+use InvalidArgumentException;
+use OutOfBoundsException;
+use Traversable;
+use Wikibase\DataModel\Entity\PropertyId;
+
+/**
+ * Maps property id providers by their property id.
+ *
+ * This class allows to reorder elements having the same property id
+ * as well as changíng the order of the property ids themselves.
+ *
+ * @since 3.0
+ *
+ * @license GNU GPL v2+
+ * @author Bene* < benestar.wikimedia@gmail.com >
+ */
+class ByPropertyIdMap {
+
+	/**
+	 * Map of serialized property ids pointing to
+	 * a list of elements with that property id
+	 *
+	 * @var array[]
+	 */
+	private $byPropertyId = array();
+
+	/**
+	 * @var PropertyId[]
+	 */
+	private $propertyIds = array();
+
+	/**
+	 * @param PropertyIdProvider[]|Traversable $propertyIdProviders
+	 *
+	 * @throws InvalidArgumentException
+	 */
+	public function __construct( $propertyIdProviders ) {
+		$byPropertyIdGrouper = new ByPropertyIdGrouper( $propertyIdProviders );
+
+		$this->propertyIds = $byPropertyIdGrouper->getPropertyIds();
+		foreach ( $this->propertyIds as $propertyId ) {
+			$this->byPropertyId[$propertyId->getSerialization()] = $byPropertyIdGrouper->getByPropertyId( $propertyId );
+		}
+	}
+
+	/**
+	 * @param PropertyId $propertyId
+	 * @param int|null $index
+	 *
+	 * @throws InvalidArgumentException
+	 * @throws OutOfBoundsException
+	 */
+	public function moveGroupToIndex( PropertyId $propertyId, $index ) {
+		$this->assertIsIndex( $index );
+
+		$oldIndex = array_search( $propertyId, $this->propertyIds );
+
+		if ( $oldIndex === false ) {
+			throw new OutOfBoundsException( 'There is no group for property id ' . $propertyId->getSerialization() );
+		}
+
+		if ( $index === null ) {
+			$index = count( $this->propertyIds );
+		}
+
+		array_splice( $this->propertyIds, $oldIndex, 1 );
+		array_splice( $this->propertyIds, $index, 0, array( $propertyId ) );
+
+		$this->propertyIds = array_values( $this->propertyIds );
+	}
+
+	/**
+	 * @param PropertyIdProvider $propertyIdProvider
+	 * @param int|null $index
+	 *
+	 * @throws InvalidArgumentException
+	 * @throws OutOfBoundsException
+	 */
+	public function moveElementToIndex( PropertyIdProvider $propertyIdProvider, $index ) {
+		$this->assertIsIndex( $index );
+
+		$idSerialization = $propertyIdProvider->getPropertyId()->getSerialization();
+
+		if ( !isset( $this->byPropertyId[$idSerialization] ) ) {
+			throw new OutOfBoundsException( 'There is no group for property id ' . $idSerialization );
+		}
+
+		$oldIndex = array_search( $propertyIdProvider, $this->byPropertyId[$idSerialization] );
+
+		if ( $oldIndex === false ) {
+			throw new OutOfBoundsException( 'The property id provider does not exist in this map' );
+		}
+
+		if ( $index === null ) {
+			$index = count( $this->byPropertyId[$idSerialization] );
+		}
+
+		array_splice( $this->byPropertyId[$idSerialization], $oldIndex, 1 );
+		array_splice( $this->byPropertyId[$idSerialization], $index, 0, array( $propertyIdProvider ) );
+
+		$this->byPropertyId[$idSerialization] = array_values( $this->byPropertyId[$idSerialization] );
+	}
+
+	/**
+	 * @param PropertyIdProvider $propertyIdProvider
+	 * @param int|null $index
+	 *
+	 * @throws InvalidArgumentException
+	 */
+	public function addElementAtIndex( PropertyIdProvider $propertyIdProvider, $index ) {
+		$this->assertIsIndex( $index );
+
+		$idSerialization = $propertyIdProvider->getPropertyId()->getSerialization();
+
+		if ( !isset( $this->byPropertyId[$idSerialization] ) ) {
+			$this->byPropertyId[$idSerialization] = array();
+			$this->propertyIds[] = $propertyIdProvider->getPropertyId();
+		}
+
+		if ( $index === null ) {
+			$index = count( $this->byPropertyId[$idSerialization] );
+		}
+
+		array_splice( $this->byPropertyId[$idSerialization], $index, 0, array( $propertyIdProvider ) );
+
+		$this->byPropertyId[$idSerialization] = array_values( $this->byPropertyId[$idSerialization] );
+	}
+
+	private function assertIsIndex( $index ) {
+		if ( ( !is_int( $index ) || $index < 0 ) && $index !== null ) {
+			throw new InvalidArgumentException( '$index must be a non-negative integer or null' );
+		}
+	}
+
+	/**
+	 * @return PropertyIdProvider[]
+	 */
+	public function getFlatArray() {
+		$propertyIdProviders = array();
+
+		foreach ( $this->propertyIds as $propertyId ) {
+			foreach ( $this->byPropertyId[$propertyId->getSerialization()] as $propertyIdProvider ) {
+				$propertyIdProviders[] = $propertyIdProvider;
+			}
+		}
+
+		return $propertyIdProviders;
+	}
+
+}
diff --git a/tests/unit/ByPropertyIdMapTest.php b/tests/unit/ByPropertyIdMapTest.php
new file mode 100644
index 00000000..1cfc9fb2
--- /dev/null
+++ b/tests/unit/ByPropertyIdMapTest.php
@@ -0,0 +1,309 @@
+<?php
+
+namespace Wikibase\Test;
+
+use DataValues\StringValue;
+use Wikibase\DataModel\ByPropertyIdMap;
+use Wikibase\DataModel\Entity\PropertyId;
+use Wikibase\DataModel\Snak\PropertyValueSnak;
+
+/**
+ * @covers Wikibase\DataModel\ByPropertyIdMap
+ *
+ * @author Bene* < benestar.wikimedia@gmail.com >
+ */
+class ByPropertyIdMapTest extends \PHPUnit_Framework_TestCase {
+
+	private function getPropertyIdProvider( $propertyId, $value ) {
+		return new PropertyValueSnak( new PropertyId( $propertyId ), new StringValue( $value ) );
+	}
+
+	private function assertTypesAreEqual( array $types, ByPropertyIdMap $byPropertyIdMap ) {
+		$flatArray = $byPropertyIdMap->getFlatArray();
+
+		array_walk( $flatArray, function( PropertyValueSnak &$snak ) {
+			$snak = $snak->getDataValue()->getValue();
+		} );
+
+		$this->assertEquals( $types, $flatArray );
+	}
+
+	public function testConstructWithGroupedList_getFlatArrayReturnsSameList() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P789', 'boo' )
+		) );
+
+		$this->assertTypesAreEqual( array( 'foo', 'bar', 'baz', 'boo' ), $byPropertyIdMap );
+	}
+
+	public function testConstructWithUngroupedList_getFlatArrayReturnsGroupedList() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P321', 'boo' )
+		) );
+
+		$this->assertTypesAreEqual( array( 'foo', 'boo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenIntegerIndex_moveGroupToIndexMovesGroup() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P789', 'boo' )
+		) );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P123' ), 2 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'boo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenZeroIndex_moveGroupToIndexMovesGroupToBeginning() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P789', 'boo' )
+		) );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P789' ), 0 );
+
+		$this->assertTypesAreEqual( array( 'boo', 'foo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenHugeIndex_moveGroupToIndexMovesGroupToEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P789', 'boo' )
+		) );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P321' ), 999999 );
+
+		$this->assertTypesAreEqual( array( 'bar', 'baz', 'boo', 'foo' ), $byPropertyIdMap );
+	}
+
+	public function testGivenNullIndex_moveGroupToIndexMovesGroupToEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P789', 'boo' )
+		) );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P321' ), null );
+
+		$this->assertTypesAreEqual( array( 'bar', 'baz', 'boo', 'foo' ), $byPropertyIdMap );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNonIntegerIndex_moveGroupToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P42' ), false );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNegativeIndex_moveGroupToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P42' ), -1 );
+	}
+
+	/**
+	 * @expectedException OutOfBoundsException
+	 */
+	public function testGivenNotExistingPropertyId_moveGroupToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveGroupToIndex( new PropertyId( 'P42' ), 0 );
+	}
+
+	public function testGivenIntegerIndex_moveElementToIndexMovesPropertyIdProvider() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P123', 'boo' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P123', 'bar' );
+		$byPropertyIdMap->moveElementToIndex( $propertyIdProvider, 1 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'baz', 'bar', 'boo' ), $byPropertyIdMap );
+	}
+
+	public function testGivenZeroIndex_moveElementToIndexMovesPropertyIdProviderToBeginning() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P123', 'boo' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P123', 'boo' );
+		$byPropertyIdMap->moveElementToIndex( $propertyIdProvider, 0 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'boo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenHugeIndex_moveElementToIndexMovesPropertyIdProviderToEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P123', 'boo' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P123', 'baz' );
+		$byPropertyIdMap->moveElementToIndex( $propertyIdProvider, 999999 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'bar', 'boo', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenNullIndex_moveElementToIndexMovesPropertyIdProviderToEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' ),
+			$this->getPropertyIdProvider( 'P123', 'boo' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P123', 'baz' );
+		$byPropertyIdMap->moveElementToIndex( $propertyIdProvider, null );
+
+		$this->assertTypesAreEqual( array( 'foo', 'bar', 'boo', 'baz' ), $byPropertyIdMap );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNonIntegerIndex_moveElementToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveElementToIndex( $this->getPropertyIdProvider( 'P42', 'foo' ), false );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNegativeIndex_moveElementToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveElementToIndex( $this->getPropertyIdProvider( 'P42', 'foo' ), -1 );
+	}
+
+	/**
+	 * @expectedException OutOfBoundsException
+	 */
+	public function testGivenNotExistingPropertyId_moveElementToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->moveElementToIndex( $this->getPropertyIdProvider( 'P42', 'foo' ), 0 );
+	}
+
+	/**
+	 * @expectedException OutOfBoundsException
+	 */
+	public function testGivenNotExistingPropertyIdProvider_moveElementToIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P42', 'foo' )
+		) );
+
+		$byPropertyIdMap->moveElementToIndex( $this->getPropertyIdProvider( 'P42', 'bar' ), 0 );
+	}
+
+	public function testGivenIntegerIndex_addElementAtIndexAddsPropertyIdProvider() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P123', 'boo' );
+		$byPropertyIdMap->addElementAtIndex( $propertyIdProvider, 1 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'bar', 'boo', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenZeroIndex_addElementAtIndexAddsPropertyIdProviderAtBeginning() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P321', 'boo' );
+		$byPropertyIdMap->addElementAtIndex( $propertyIdProvider, 0 );
+
+		$this->assertTypesAreEqual( array( 'boo', 'foo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenHugeIndex_addElementAtIndexAddsPropertyIdProviderAtEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P321', 'boo' );
+		$byPropertyIdMap->addElementAtIndex( $propertyIdProvider, 999999 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'boo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenNullIndex_addElementAtIndexAddsPropertyIdProviderAtEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P321', 'boo' );
+		$byPropertyIdMap->addElementAtIndex( $propertyIdProvider, null );
+
+		$this->assertTypesAreEqual( array( 'foo', 'boo', 'bar', 'baz' ), $byPropertyIdMap );
+	}
+
+	public function testGivenNotExistingPropertyId_addElementAtIndexCreatesNewGroupAtEnd() {
+		$byPropertyIdMap = new ByPropertyIdMap( array(
+			$this->getPropertyIdProvider( 'P321', 'foo' ),
+			$this->getPropertyIdProvider( 'P123', 'bar' ),
+			$this->getPropertyIdProvider( 'P123', 'baz' )
+		) );
+
+		$propertyIdProvider = $this->getPropertyIdProvider( 'P456', 'boo' );
+		$byPropertyIdMap->addElementAtIndex( $propertyIdProvider, 1 );
+
+		$this->assertTypesAreEqual( array( 'foo', 'bar', 'baz', 'boo' ), $byPropertyIdMap );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNonIntegerIndex_addElementAtIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->addElementAtIndex( $this->getPropertyIdProvider( 'P42', 'foo' ), false );
+	}
+
+	/**
+	 * @expectedException InvalidArgumentException
+	 */
+	public function testGivenNegativeIndex_addElementAtIndexThrowsException() {
+		$byPropertyIdMap = new ByPropertyIdMap( array() );
+
+		$byPropertyIdMap->addElementAtIndex( $this->getPropertyIdProvider( 'P42', 'foo' ), -1 );
+	}
+
+}