1-field.php

        $this->add(
            array(
                'type' => 'DoctrineModule\Form\Element\ObjectSelect',
                'name' => 'field_id',
                'options' => array(
                    'object_manager' => $em,
                    'target_class'   => 'Application\Entity\Field',
                    'label_generator' => function($targetEntity) {
                        return $targetEntity->getName();
                    },
                    'find_method'    => array(
                        'name'   => 'findCurrentMainpositionFields',
                        'params' => array('calendar_id' => $calendar->getCalendarId())
                    ),
                ),
                'attributers' => array(
                    'id' => 'position',
                    'class'=> 'fake-select'
                )
            )
        );

2-Field-Entity.php

<?php

namespace Application\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Application\Entity\Field
 *
 * @ORM\Entity(repositoryClass="Application\Repository\FieldRepository")
 * @ORM\Table(name="`field`", indexes={@ORM\Index(name="fk_field_tab1_idx", columns={"tab_id"}), @ORM\Index(name="fk_field_field1_idx", columns={"parent_id"}), @ORM\Index(name="fk_field_versionset1_idx", columns={"versionset_id"})})
 */
class Field
{
    /**
     * @ORM\Id
     * @ORM\Column(name="`field_id`", type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $field_id;

    /**
     * @ORM\Column(name="`name`", type="string", length=100)
     */
    protected $name;

    /**
     * @ORM\Column(name="`color`", type="string", length=6, nullable=true)
     */
    protected $color;

    /**
     * title, meta, calc
     *
     * @ORM\Column(name="`type`", type="string", length=20)
     */
    protected $type;

    /**
     * @ORM\Column(name="`pos`", type="integer")
     */
    protected $pos;

    /**
     * @ORM\Column(name="`weighting`", type="float", precision=4, scale=2, nullable=true)
     */
    protected $weighting;

    /**
     * step for slider
     *
     * @ORM\Column(name="`step`", type="integer", nullable=true)
     */
    protected $step;

    /**
     * @ORM\OneToMany(targetEntity="Event", mappedBy="field")
     * @ORM\JoinColumn(name="field_id", referencedColumnName="field_id", nullable=false)
     */
    protected $events;

    /**
     * @ORM\OneToMany(targetEntity="Rating", mappedBy="field")
     * @ORM\JoinColumn(name="field_id", referencedColumnName="field_id", nullable=false)
     */
    protected $ratings;

    /**
     * @ORM\ManyToOne(targetEntity="Tab", inversedBy="fields")
     * @ORM\JoinColumn(name="tab_id", referencedColumnName="tab_id", nullable=false)
     */
    protected $tab;

    /**
     * @ORM\ManyToOne(targetEntity="Versionset", inversedBy="fields")
     * @ORM\JoinColumn(name="versionset_id", referencedColumnName="versionset_id", nullable=false)
     */
    protected $versionset;

    /**
     * @ORM\ManyToOne(targetEntity="Field")
     * @ORM\JoinColumn(name="parent_id", referencedColumnName="field_id")
     */
    protected $field;

    public function __construct()
    {
        $this->events = new ArrayCollection();
        $this->ratings = new ArrayCollection();
    }

    /**
     * Set the value of field_id.
     *
     * @param integer $field_id
     * @return \Application\Entity\Field
     */
    public function setFieldId($field_id)
    {
        $this->field_id = $field_id;

        return $this;
    }

    /**
     * Get the value of field_id.
     *
     * @return integer
     */
    public function getFieldId()
    {
        return $this->field_id;
    }

    /**
     * Set the value of name.
     *
     * @param string $name
     * @return \Application\Entity\Field
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get the value of name.
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Set the value of color.
     *
     * @param string $color
     * @return \Application\Entity\Field
     */
    public function setColor($color)
    {
        $this->color = $color;

        return $this;
    }

    /**
     * Get the value of color.
     *
     * @return string
     */
    public function getColor()
    {
        return $this->color;
    }

    /**
     * Set the value of type.
     *
     * @param string $type
     * @return \Application\Entity\Field
     */
    public function setType($type)
    {
        $this->type = $type;

        return $this;
    }

    /**
     * Get the value of type.
     *
     * @return string
     */
    public function getType()
    {
        return $this->type;
    }

    /**
     * Set the value of pos.
     *
     * @param integer $pos
     * @return \Application\Entity\Field
     */
    public function setPos($pos)
    {
        $this->pos = $pos;

        return $this;
    }

    /**
     * Get the value of pos.
     *
     * @return integer
     */
    public function getPos()
    {
        return $this->pos;
    }

    /**
     * Set the value of weighting.
     *
     * @param float $weighting
     * @return \Application\Entity\Field
     */
    public function setWeighting($weighting)
    {
        $this->weighting = $weighting;

        return $this;
    }

    /**
     * Get the value of weighting.
     *
     * @return float
     */
    public function getWeighting()
    {
        return $this->weighting;
    }

    /**
     * Set the value of step.
     *
     * @param integer $step
     * @return \Application\Entity\Field
     */
    public function setStep($step)
    {
        $this->step = $step;

        return $this;
    }

    /**
     * Get the value of step.
     *
     * @return integer
     */
    public function getStep()
    {
        return $this->step;
    }

    /**
     * Add Event entity to collection (one to many).
     *
     * @param \Application\Entity\Event $event
     * @return \Application\Entity\Field
     */
    public function addEvent(Event $event)
    {
        $this->events[] = $event;

        return $this;
    }

    /**
     * Get Event entity collection (one to many).
     *
     * @return \Doctrine\Common\Collections\Collection
     */
    public function getEvents()
    {
        return $this->events;
    }

    /**
     * Add Rating entity to collection (one to many).
     *
     * @param \Application\Entity\Rating $rating
     * @return \Application\Entity\Field
     */
    public function addRating(Rating $rating)
    {
        $this->ratings[] = $rating;

        return $this;
    }

    /**
     * Get Rating entity collection (one to many).
     *
     * @return \Doctrine\Common\Collections\Collection
     */
    public function getRatings()
    {
        return $this->ratings;
    }

    /**
     * Set Tab entity (many to one).
     *
     * @param \Application\Entity\Tab $tab
     * @return \Application\Entity\Field
     */
    public function setTab(Tab $tab = null)
    {
        $this->tab = $tab;

        return $this;
    }

    /**
     * Get Tab entity (many to one).
     *
     * @return \Application\Entity\Tab
     */
    public function getTab()
    {
        return $this->tab;
    }

    /**
     * Set Versionset entity (many to one).
     *
     * @param \Application\Entity\Versionset $versionset
     * @return \Application\Entity\Field
     */
    public function setVersionset(Versionset $versionset = null)
    {
        $this->versionset = $versionset;

        return $this;
    }

    /**
     * Get Versionset entity (many to one).
     *
     * @return \Application\Entity\Versionset
     */
    public function getVersionset()
    {
        return $this->versionset;
    }

    /**
     * Set Field entity (many to one).
     *
     * @param \Application\Entity\Field $field
     * @return \Application\Entity\Field
     */
    public function setField(Field $field = null)
    {
        $this->field = $field;

        return $this;
    }

    /**
     * Get Field entity (many to one).
     *
     * @return \Application\Entity\Field
     */
    public function getField()
    {
        return $this->field;
    }

    public function __sleep()
    {
        return array('field_id', 'tab_id', 'versionset_id', 'name', 'color', 'type', 'parent_id', 'pos', 'weighting', 'step');
    }
}

3-error.txt

'INSERT INTO `event` (`name`, `shortname`, `date_start`, `date_end`, `state`, `department`, `responsible`, `comment`, `evaluated`, calendar_id, versionset_id, field_id) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)' with params ["SuperDuper", "super", "2013-05-01", "2013-05-15", "2", "department", "responsible", "comment", 0, 1, "8", null]:

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'field_id' cannot be null

4-post.txt

Array
(
    [event] => Array
        (
            [event_id] => 
            [calendar_id] => 
            [name] => SuperDuper
            [shortname] => super
            [date_start_display] => 01.05.2013
            [date_start] => 2013-05-01
            [date_end_display] => 15.05.2013
            [date_end] => 2013-05-15
            [milestones] => Array
                (
                    [0] => Array
                        (
                            [event_id] => 
                            [milestone_id] => 
                            [milestones_display_from] => 
                            [date_start] => 
                            [milestones_display_to] => 
                            [date_end] => 
                        )

                )

            [field_id] => 49
            [state] => 2
            [department] => 
            [responsible] => 
            [comment] => 
        )

    [calendar] => 1
)