How to run a nextflow process for each file generated by another process separately - Input tuple does not match input set cardinality error

Question

I have a process called FINDTAIL that generates different number of files depending on the input data. Its either 2 files or four files i.e.

1. read_1{,.pr,.sl}.fasta and read_2{,.pr,.sl}.fasta

or

2. read_1.pr.fasta, read_1.sl.fasta, read_2.pr.fasta and read_2.sl.fasta

/*
 * the `FINDTAIL` process takes "${pair_id}_strand_check.txt" file  
 * and executes commands according to the reads strandedness. 
 * The output generated is FASTA files.  
 */
 
process FINDTAIL {

    tag { pair_id }

    debug true

    input:
    path strand_check_file
    tuple val(pair_id), path(reads)

    output:
    tuple val(pair_id), path("tail_${pair_id}_{1,2}{,.pr,.sl}.fasta")
    
    script:
    """
#!/usr/bin/env python3

import subprocess
import pandas as pd
import os


result = pd.read_csv("${strand_check_file}", sep="\\r\\n", header=None, engine='python')

failed = float(result.iloc[1,0].replace('Fraction of reads failed to determine: ', ''))
fwd = float(result.iloc[2,0].replace('Fraction of reads explained by "1++,1--,2+-,2-+": ', ''))
rev = float(result.iloc[3,0].replace('Fraction of reads explained by "1+-,1-+,2++,2--": ', ''))
fwd_percent = fwd/(fwd+rev)
rev_percent = rev/(fwd+rev)

if float(result.iloc[1,0].replace('Fraction of reads failed to determine: ', '')) > 0.50:
    cmd1 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_1.sl.fasta"
    cmd2 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_2.sl.fasta"
    print(cmd1)
    subprocess.call(cmd1, shell=True)
    print(cmd2)
    subprocess.call(cmd2, shell=True)

if fwd_percent > 0.9:
    #Over 90% of reads explained by "1++,1--,2+-,2-+
    #Data is likely FR/fr-secondstrand
    cmd1 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type pr --d > " + "tail_${pair_id}_1.pr.fasta"
    cmd2 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type pr --d > " + "tail_${pair_id}_2.pr.fasta"
    print(cmd1)
    subprocess.call(cmd1, shell=True)
    print(cmd2)
    subprocess.call(cmd2, shell=True)


elif rev_percent > 0.9:
    # Over 90% of reads explained by "1+-,1-+,2++,2--
    # Data is likely RF/fr-firststrand
    cmd1 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_1.sl.fasta"
    cmd2 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_2.sl.fasta"
    print(cmd1)
    subprocess.call(cmd1, shell=True)
    print(cmd2)
    subprocess.call(cmd2, shell=True)

elif max(fwd_percent, rev_percent) < 0.6:
    #Under 60% of reads explained by one direction
    #Data is likely unstranded
    cmd1 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_1.sl.fasta"
    cmd2 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type pr --d > " + "tail_${pair_id}_1.pr.fasta"
    cmd3 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_2.sl.fasta"
    cmd4 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type pr --d > " + "tail_${pair_id}_2.pr.fasta"
    print(cmd1)
    subprocess.call(cmd1, shell=True)
    print(cmd2)
    subprocess.call(cmd2, shell=True)
    print(cmd3)
    subprocess.call(cmd3, shell=True)
    print(cmd4)
    subprocess.call(cmd4, shell=True)

else:
    #if strand couldnt be detected
    # we assume it is first-stranded
    # since most illumina reads are
    cmd1 = "findtail_v1.01 --input_file " + "${reads[0]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_1.sl.fasta"
    cmd2 = "findtail_v1.01 --input_file " + "${reads[1]}" + " --seqlength 500 --endgap 2 --taillength 10 --identity 10 --ptype A --stype T --output_format fasta --output_type sl --d > " + "tail_${pair_id}_2.sl.fasta"
    print(cmd1)
    subprocess.call(cmd1, shell=True)
    print(cmd2)
    subprocess.call(cmd2, shell=True)
    """
}

I want to pass these fasta file outputs of the FINDTAIL process into an ALIGN process that runs on each file separately.

The ALIGN process:

/*
 * define the `ALIGN` process that outputs unmapped files in FASTA format
 * It will take the FASTA files creates by the `FINDTAIL` process 
 * and try to align them INDIVIDUALLY.
 */

fasta_file_ch = find_tail_ch.flatten()
 
process ALIGN {

    tag { pair_id }

    input:
    path index_files, stageAs: "bt2_index/*"
    tuple val(pair_id)
    file(fasta_file) from fasta_file_ch

    output:
    tuple val(pair_id), path("unmapped_tail_${pair_id}_{1,2}{,.pr,.sl}.fasta")

    script:
    def idx = index_files[0].getBaseName(2)    

    """
    bowtie2 -x "${idx}" -f "${fasta_file}" --un "unmapped_tail_${pair_id}/${it.baseName}.fasta"
    """
}

I have tried using the flatten() function to format my find_tail_ch but it still does not work.

Best wishes~

Answer 1

If the output channel produces a tuple where the first element is a simple value and the second element is a collection of files, like in this case, you can simply use the transpose operator to produce a transposition where the first element in each resulting tuple is the pair_id and the second is a FASTA file. You'll then need to update your process definition to receive this tuple, for example:

process ALIGN {

    tag { pair_id }

    input:
    path index_files, stageAs: "bt2_index/*"
    tuple val(pair_id), path(fasta_file)

    output:
    tuple val(pair_id), path("${fasta_file.baseName}.unmapped.fasta")

    script:
    def idx = index_files[0].getBaseName(2)    

    """
    bowtie2 \\
        -x "${idx}" \\
        -f "${fasta_file}" \\
        --un "${fasta_file.baseName}.unmapped.fasta"
    """
}

Your workflow block might look like:

workflow {

    bt2_index = ...

    ...

    align_input_ch = find_tail_ch.transpose()

    ALIGN( bt2_index, align_input_ch )
}