SonataAdminBundle: FatalErrorException: Error: Call to a member function add() on a non-object...

The problem usually occurs during updating a data record in the admin/dashboard of SonataAdminBundle.
The cause of the problem is that the add() method, defined in "vendor/sonata-project/doctrine-orm-admin-bundle/Sonata/DoctrineORMAdminBundle/Model/ModelManager.php line 560" received a two dimensional array object instead of a simple array.
Let's take a look at the User.php:

//src/Enstb/Bundle/VisplatBundle/Entity/User.php
public function getRoles()
    {
        return $this->roles->toArray();
    }

The getRoles method is automatically called when updating a user. As you can see, it wraps the roles object with an array, this is required for authentication system of Symfony.
Well, what we can do? One of the possible solutions is making a new name for roles, in our case is rolesCollection, to let us configure a different return type.

//src/Enstb/Bundle/VisplatBundle/Administrator/UserAdmin.php
protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            //...
            ->add('rolesCollection','entity',array(
                'class' => 'EnstbVisplatBundle:Role',
                'property'=>'name',
                'expanded' => true,
                'compound' => true,
                'multiple' => true,
                'by_reference' => false
            ));
    }

Then, the add,remove,get, and set are required:

//src/Enstb/Bundle/VisplatBundle/Entity/User.php
 public function addRolesCollection($role){
        $this->addRole($role);
    }
public function removeRolesCollection($role){
        $this->removeRole($role);
    }
public function getRolesCollection()
    {
        return $this->roles;
    }
public function setRolesCollection($roles)
    {
        if (count($roles) > 0) {
            foreach ($roles as $role) {
                $this->addRole($role);
            }
        }
        return $this;
    }

Finally, DONT forget to verify an existing roles before adding it in order to prevent duplicating roles added into a database.

//src/Enstb/Bundle/VisplatBundle/Entity/User.php
public function addRole(\Enstb\Bundle\VisplatBundle\Entity\Role $role)
    {
        if(!in_array($role,$this->getRoles())){
            // Link each role with the user
            $role->addUser($this);
            $this->roles->add($role);
        }

        return $this;
    }

SonataAdminBundle forms One to Many or Many to Many does not persist

This problem occurs due to the inverse-side object is not defined in the owning-side. In order to solve this, we have to manually specify it in the owning-side class. Suppose that we have a user class and a role class that are inverse-side and owning-side respectively. Here is the example of defining the inverse-side object.

//Acme/Bundle/DemeBundle/Entity\User
public function addRole(\Enstb\Bundle\VisplatBundle\Entity\Role $role)
{
    // Link each role with the user 
    // (This is important for SonataAdminBundle!!)
    $role->addUser($this);
    $this->roles->add($role);
    return $this;
}
public function removeRole(\Enstb\Bundle\VisplatBundle\Entity\Role $role)
{
    // Link each role with the user
    $role->removeUser($this);
    $this->roles->removeElement($role);
}
public function getRoles()
{
    return $this->roles;
}

The most important part is $role->addUser($this). It sends the object of Users class to be added in the Roles class. This makes the SonataAdminBundle to realize that which object should be linked together and persisted to a database.
And here is an example of addUser method in the Roles class:

//Acme/Bundle/DemeBundle/Entity\Role
public function addUser(\Enstb\Bundle\VisplatBundle\Entity\Users $user)
{
    $this->Users->add($user);
    return $this;
} 

Note: if you are not sure what is owning-side and inverse-side, read more about What is the difference between inversedBy and mappedBy?.

SonataAdminBundle - Making fields for two entities (Many to many relation)

If you always got the "Catchable Fatal Error: Object of class Acme\Bundle\DemoBundle\Entity\Roles could not be converted to string.", this tutorial will provide you a way to deal with it.
First of all, the error is returned because the __toString() class is not overridden. So, let make it! 

    //Acme\Bundle\DemoBundle\Entity\Roles
    function __toString()
    {
        return $this->getName();
    }

In order to make a checkbox of the roles available, we should specify true values for  expanded, compound, and multiple

   //Acme\Bundle\DemoBundle\Admin\UsersAdmin
    protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            ->add('Roles','sonata_type_model',array(
                  'expanded' => true,
                  'compound' => true,
                  'multiple' => true))
    ;    }