Doctrine\ODM\MongoDB\Mapping\ClassMetadataInfo::getFieldMapping PHP Method

    public function getFieldMapping($fieldName)
    {
        if (!isset($this->fieldMappings[$fieldName])) {
            throw MappingException::mappingNotFound($this->name, $fieldName);
        }
        return $this->fieldMappings[$fieldName];
    }

 public function testDefaultDiscriminatorField()
 {
     $cm = new ClassMetadataInfo('stdClass');
     $cm->mapField(array('fieldName' => 'assoc', 'reference' => true, 'type' => 'one'));
     $cm->mapField(array('fieldName' => 'assocWithTargetDocument', 'reference' => true, 'type' => 'one', 'targetDocument' => 'stdClass'));
     $cm->mapField(array('fieldName' => 'assocWithDiscriminatorField', 'reference' => true, 'type' => 'one', 'discriminatorField' => 'type'));
     $mapping = $cm->getFieldMapping('assoc');
     $this->assertEquals(ClassMetadataInfo::DEFAULT_DISCRIMINATOR_FIELD, $mapping['discriminatorField'], 'Default discriminator field is set for associations without targetDocument and discriminatorField options');
     $mapping = $cm->getFieldMapping('assocWithTargetDocument');
     $this->assertArrayNotHasKey('discriminatorField', $mapping, 'Default discriminator field is not set for associations with targetDocument option');
     $mapping = $cm->getFieldMapping('assocWithDiscriminatorField');
     $this->assertEquals('type', $mapping['discriminatorField'], 'Default discriminator field is not set for associations with discriminatorField option');
 }