Symfony2 - Only one field has its value read, the rest of this field is NULL when submitting form

I have two related tables: material and items_budget . The items_budget table has a form that has a field that lists the name of all the material in a group of checkbox , and next to each checkbox has two % fields, one for quantity and one for price . Below is the code for listing these Material

<strong>Materiais</strong>
{% for material in materials %}
    <div class="checkbox">
        <label>
            <input type="checkbox" class="itemsbudget_material" name="cdg_itemsbudget_type[material]" value="{{ material.id }}"> {{ material.name }} - 
        </label>
        <input type="hidden" class="itemsbudget_price_hidden" value="{{ material.price }}"/>
        <input type="text" class="itemsbudget_quantity" name="cdg_itemsbudget_type[quantity]" placeholder="Qtd" size="3"/>x - R$
        <input type="text" class="itemsbudget_price" name="cdg_itemsbudget_type[price]" value="0" size="3" readonly/>
    </div>
{% endfor %}

I also have a trigger that runs every time new data is persisted in the items_budget table. It subtracts the quantity from the currentQuantity of the material . It works perfectly, but now comes the problem: it just works for the first record of the material table , because for others the quantity mysteriously gets NULL after submitting the form

I have tried to add a couple of brackets at the end of each attribute input , but by doing so, the form is simply reloaded and no data is persisted.

I really want to make this as clear as possible, so I put together an image of the steps I followed. From top left to bottom right: I first checked the option that matches the first record of the material table, I entered with a random value for quantity and so I submitted the form. According to the second image, everything worked normally. But now, the other two images demonstrate the problem. I checked another option and entered again with a quantity value, but this time the field becomes NULL, as can be seen in the last image.

Here, only the name class and its ItemsBudgetController method:

public function addAction(Request $request)
{
    $form = $this->createForm(new ItemsBudgetType());
    $manager = $this->getDoctrine()->getManager();

    if ($request->getMethod() == 'POST') {
        $form->handleRequest($request);

        if ($form->isValid()) {
            $ItemsBudgetEntity = $form->getData();
            $manager->persist($ItemsBudgetEntity);
            $manager->flush();

            $BudgetEntity = $form->get('budget')->getData();
            $BudgetEntity->addItemsBudget($ItemsBudgetEntity);

            $manager->persist($BudgetEntity);
            $manager->flush();

            $this->addFlash('success', 'Materiais para o orçamento adicionados');

            return $this->redirect($this->generateUrl('panel_budgets'));
        }
    }

    return $this->render('PanelBundle:ItemsBudget:index.html.twig', array(
        'budgets' => $manager->getRepository('PanelBundle:Budget')->findAll(),
        'materials' => $manager->getRepository('PanelBundle:Material')->findAll()
    ));
}

And now, addAction , where I set the ItemsBudgetType field as an entity:

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('budget', 'entity', array(
                'class' => 'PanelBundle:Budget',
                'attr' => array(
                    'class' => 'form-control',
                ),
            ))
            ->add('material', 'entity', array(
                'class' => 'PanelBundle:Material',
                'attr' => array(
                    'required' => true,
                ),
            ))
            ->add('quantity', 'number', array(
                'attr' => array(
                    'class' => 'form-control',
                ),
            ))
            ->add('price', 'money', array(
                'attr' => array(
                    'class' => 'form-control',
                ),
            ));
}

This does not make sense. Why only the first record has its quantity field seen? If you need me to post more codes, I'll update the question.

Issue # 1

I'll show the relationship between tables. When checking how to make a field as a collection with an entity, I understood that it only works with manyToMany relationships.

First, material :

CDG\PanelBundle\Entity\Material:
    type: entity
    table: material
    repositoryClass: CDG\PanelBundle\Entity\MaterialRepository
    id:
        id:
            type: integer
            id: true
            generator:
                strategy: AUTO
    fields:
        name:
            type: string
            length: 255
        description:
            type: string
            length: 255
        currentQuantity:
            type: integer
            column: current_quantity
        price:
            type: decimal
        dateCreated:
            type: datetime
    oneToMany:
        itemsBudget:
            targetEntity: ItemsBudget
            mappedBy: material
        quantityMaterial:
            targetEntity: QuantityMaterial
            mappedBy: material
    lifecycleCallbacks: {  }

Then, Material :

CDG\PanelBundle\Entity\ItemsBudget:
    type: entity
    table: items_budget
    repositoryClass: CDG\PanelBundle\Entity\ItemsBudgetRepository
    id:
        id:
            type: integer
            id: true
            generator:
                strategy: AUTO
    fields:
        quantity:
            type: integer
            nullable: true
        price:
            type: decimal
    manyToOne:
        budget:
            targetEntity: Budget
            inversedBy: itemsBudget
            joinColumn:
                name: budget
                referencedColumnName: id
        material:
            targetEntity: Material
            inversedBy: itemsBudget
            joinColumn:
                name: material
                referencedColumnName: id
    lifecycleCallbacks: {  }