Fix wellplate import

lib/export/export_collections.rb

@@ -111,10 +111,6 @@ module Export
           :molecule_id => 'Molecule',
           :user_id => 'User'
         })
-        fetch_one(sample.well, {
-          :sample_id => 'Sample',
-          :wellplate_id => 'Wellplate',
-        })
         fetch_many(sample.residues, {
           :sample_id => 'Sample',
         })
@@ -173,6 +169,11 @@ module Export
 
       # fetch containers and attachments
       collection.wellplates.each do |wellplate|
+        fetch_many(wellplate.wells, {
+          :sample_id => 'Sample',
+          :wellplate_id => 'Wellplate',
+        })
+
         fetch_containers(wellplate)
       end
     end

lib/import/import_collections.rb

@@ -221,7 +221,7 @@ module Import
           'updated_at'
         ).merge({
           :wellplate => @instances.fetch('Wellplate').fetch(fields.fetch('wellplate_id')),
-          :sample => @instances.fetch('Sample').fetch(fields.fetch('sample_id'))
+          :sample => @instances.fetch('Sample').fetch(fields.fetch('sample_id'), nil)
         }))
 
         # add reaction to the @instances map
@@ -433,10 +433,13 @@ module Import
     # Follows a has_many relation to `foreign_type` through `association_type`
     def fetch_many(foreign_type, association_type, local_field, foreign_field, local_id)
       associations = []
-      @data.fetch(association_type).each do |uuid, fields|
+      @data.fetch(association_type, {}).each do |uuid, fields|
         if fields.fetch(local_field) == local_id
           foreign_id = fields.fetch(foreign_field)
-          associations << @instances.fetch(foreign_type).fetch(foreign_id)
+          instance = @instances.fetch(foreign_type, {}).fetch(foreign_id, nil)
+          unless instance.nil?
+            associations << instance
+          end
         end
       end
       return associations