def is_datastore_field(value, context):
    '''
    Make sure this field is an actual datastore field

    :raises: ckan.lib.navl.dictization_functions.Invalid for other
        inputs or non-whole values
    '''
    fields = get_datastore_fields(context['resource'].id)
    for field in fields:
        if field['id'] in value:
            return value
    raise Invalid(_('Field {0} not in datastore'.format(value)))
    def setup_template_variables(self, context, data_dict):
        """Setup variables available to templates"""

        records_per_page = config.get("ckanext.gallery.records_per_page", 32)
        current_page = request.params.get('page', 1)
        image_list = []

        # Get list of datastore fields and format into a dict, ready for a form
        datastore_fields = get_datastore_fields(data_dict['resource']['id'])

        image_plugins = []
        # Get gallery image plugins
        for plugin in p.PluginImplementations(IGalleryImage):
            image_info = plugin.image_info()
            # If we have resource type set, make sure the format of the resource matches
            # Otherwise continue to next record
            if image_info['resource_type'] and data_dict['resource']['format'].lower() not in image_info['resource_type']:
                continue
            image_info['plugin'] = plugin
            image_plugins.append(image_info)

        image_field = data_dict['resource_view'].get('image_field', None)
        image_plugin = data_dict['resource_view'].get('image_plugin', None)
        image_title_field = data_dict['resource_view'].get('image_title', None)
        # Set up template variables
        tpl_variables = {
            'images': image_list,
            'datastore_fields': [{'value': f['id'], 'text': f['id']} for f in datastore_fields],
            'image_plugins': [{'value': f['plugin'].name, 'text': f['title']} for f in image_plugins],
            'defaults': {},
            'resource_id': data_dict['resource']['id'],
            'package_name': data_dict['package']['name'],
            'has_title': True if image_title_field else False
        }
        # Load the images
        # Only try and load images, if an image field has been selected
        if image_field and image_plugin:
            offset = (int(current_page) - 1) * records_per_page
            # We only want to get records that have both the image field populated
            # So add filters to the datastore search params
            filters = self._get_request_filters()
            # TODO - This means the module will only work with NHM extension
            # Need to copy across the NHM code that handles this
            filters['_has_image'] = ['true']
            params = {
                'resource_id': data_dict['resource']['id'],
                'limit': records_per_page,
                'offset': offset,
                'filters': filters,
                'sort': '_id'
            }
            # Add the full text filter
            fulltext = request.params.get('q')
            if fulltext:
                params['q'] = fulltext
            # Try and use the solr search if it exists
            try:
                search_action = toolkit.get_action('datastore_solr_search')
            # Otherwise fallback to default
            except KeyError:
                search_action = toolkit.get_action('datastore_search')
            # Perform the actual search
            context = {'model': model, 'session': model.Session, 'user': c.user or c.author}
            data = search_action(context, params)
            item_count = data.get('total', 0)
            # Get the selected gallery image plugin
            plugin = p.get_plugin(image_plugin)
            if plugin:
                for record in data['records']:
                    image_title = record.get(image_title_field, "")
                    image_defaults = {
                        'title': image_title,
                        'record_id': record['_id'],
                        'description': ''
                    }
                    field_value = record.get(image_field, None)
                    if field_value:
                        images = plugin.get_images(field_value, record, data_dict)
                        for image in images:
                            image_default_copy = copy.copy(image_defaults)
                            # Merge in the plugin image settings to the default image
                            # Using an copy so the defaults do not change for multiple images
                            image_default_copy.update(image)
                            image_list.append(image_default_copy)

            page_params = {
                'collection': data['records'],
                'page': current_page,
                'url': self.pager_url,
                'items_per_page': records_per_page,
                'item_count': item_count,
            }
            # Add filter params to page links
            for key in ['q', 'filters']:
                value = request.params.get(key)
                if value:
                    page_params[key] = value

            tpl_variables['page'] = h.Page(**page_params)

        return tpl_variables