Hi Ahmed

** Update: this is the original answer for use an onchange for the datas field from the attachment

The onchange need to be defined for the field image_logo_attachment_id in the view like:

<field name="image_logo_attachment_id" on_change="onchange_get_datas(image_logo_attachment_id)"/>

And the onchange definition in python need to be as follow:

def onchange_get_datas(self, cr, uid, ids, image_logo_attachment_id, context=None):
    if image_logo_attachment_id:
        image_logo_datas = self.pool.get('ir.attachment').browse(cr, uid, image_logo_attachment_id).datas
    else:
        image_logo_datas = False

    return {'value':{'image_logo_datas':image_logo_datas}}

****

** Update: For using directly related fields to create an attachment:

class custom_class(models.Model):
    _name = 'custom.module'

    _columns = {
        'image_logo_attachment_id': fields.many2one(
            'ir.attachment', 
            string='Image logo attachment',
            help='Technical field to store image in filestore'
        ),
        'image_logo_datas': fields.related(
            'image_logo_attachment_id', 
            'datas', 
            type='binary',
            string='Image logo datas' 
        ),
        'image_logo_datas_fname': fields.related(
            'image_logo_attachment_id', 
            'datas_fname', 
            type='char',
            string='Image logo fname'
        )
    }

    def create(self, cr, uid, vals, context=None):
        if vals.get('image_logo_datas', False) and vals.get('image_logo_datas_fname', False):
            attach_id = self.pool.get('ir.attachment').create(cr, uid, {
                'name': vals.get('image_logo_datas_fname'),
                'datas_fname': vals.get('image_logo_datas_fname'),
                'datas': vals.get('image_logo_datas')
            }, context=context)
            vals['image_logo_attachment_id'] = attach_id
        res = super(custom_class, self).create(cr, uid, vals, context=context)
        return res
    
    def write(self, cr, uid, ids, vals, context=None):
        if vals.get('image_logo_datas', False) and vals.get('image_logo_datas_fname', False):
            for elem in self.browse(cr, uid, ids, context=context):
                if not elem.image_logo_attachment_id:
                    attach_id = self.pool.get('ir.attachment').create(cr, uid, {
                        'name': vals.get('image_logo_datas_fname'),
                        'datas_fname': vals.get('image_logo_datas_fname'),
                        'datas': vals.get('image_logo_datas')
                    }, context=context)
                    elem.write({'image_logo_attachment_id': attach_id})
        res = super(custom_class, self).write(cr, uid, ids, vals, context=context)
        return res

and use it like this in the view:

<field name="image_logo_datas" filename="image_logo_datas_fname" widget='image' class="oe_avatar"/>
<field name="image_logo_datas_fname" invisible="1"/>

For the future, make sure that you provide all the code that you have to make it better to fix it and use the same references and objects names. Provide better descriptions of the situation and what you are trying to do, don't let anything out.

That make it easy to answer questions.  

**