collections_force_mapping_over.md

---

title: Force a tool to process collection jobs one by one instead of using the collection as input
area: collections
box_type: tip
layout: faq
contributors: [paulzierep, Smkingsize]

---

To map over or not to map over

Galaxy tools can either use a collection as a single input or map over a collection to process each element individually. Whether a collection is consumed as a whole or mapped over depends entirely on how the tool is designed. For example, {% tool fastp %} processes each FASTQ file individually. When you supply a collection to fastp, Galaxy indicates that: The supplied input will be mapped over this tool.


Other tools can process an entire collection at once. For example {% tool collection_column_join %} operates on all files in the collection together.


The catch

Some tools allow a collection as a single input, but you may want them to process each element one-by-one instead. A common example is {% tool metaspades %}: If you supply a collection of FASTA files, the tool will treat them as a single dataset and perform co-assembly, which is its default behavior. However, in many workflows you want to assemble each sample individually, not all together.

Because the tool form does not offer an option to switch this behavior, you can force mapping-over by creating a nested list.

Solution - Create a nested collection

Convert your original collection into a collection of collections (list:list:). This forces any tool - including ones that normally process the whole collection - to run on each subcollection individually.

Solution for Galaxy Server with Version > 25.1: Create a nested collection using Nest collection

Use the {% tool Nest collection %} tool to convert your original collection into a collection of collections.

Solution for Galaxy Server with Version < 25.1: Create a nested collection using APPLY_RULES

Use the {% tool Apply rules %} tool to convert your original collection into a collection of collections.

1. Open {% tool Apply rules %}

2. Select your collection

3. Click Edit

Adding another nesting level to create list:list requires a few changes:

1. From Column menu select Basename of Path of URL

   • From Column A

   • Click Apply This creates a new Column B with the same list identifier as Column A.

2. From Rules menu select Add / Modify Column Definitions

   • Click Add Definition button and select List Identifier(s)

     • "Select a column": A

     • "... Assign Another Column": B

   • Click Apply

3. Click Save

4. Click Run Tool

The rule logic in a workflow editor to follow for paired reads in the Apply rules tool looks like this: