"$convert" fails to work in aggregation pipeline - mongodb

Question

I'm trying to create an aggregation pipeline wherein I need to aggregate the difference between 2 fields, and hence I try to apply a convert operation on string fields in the pipeline.
the document structure is as follows :
{
page_start : (String),
page_end : (String),
references : (list of strings)
}
match_1_stage = { "$match":{
"references":{"$exists":True},
"page_start": {"$exists": True},
"page_end": {"$exists": True}
}
}
pageConversionStage = {
"$addFields": {
"convertedPageStart": { "$convert": { "input": "page_start", "to": "decimal", "onError": "-1", "onNull": "-1" } },
"convertedPageEnd": { "$convert": { "input": "page_end", "to": "decimal", "onError": "-1", "onNull": "-1" } }
}
}
match_2_stage = { "$match":{
"convertedPageStart": {"$ne": "-1"},
"convertedPageEnd": {"$ne": "-1"}
}
}
project_stage = { "$project":{
"convertedPageStart":1,
"convertedPageEnd":1,
"no_of_pages":{ "$subtract": ["convertedPageEnd","convertedPageStart"] }
}
}
match_3_stage = { "$match":{
"no_of_pages":{ "$lt":100000 },
"no_of_pages":{ "$gte":0 },
}
}
group_stage = { "$group":{
"_id": "no_of_pages",
"totalreferences": { "$sum": { "$size": "$references" } }
}
}
res_set = articles.aggregate([match_1_stage, pageConversionStage, match_2_stage, project_stage, match_3_stage, group_stage])
The error I get is Failed to optimize pipeline :: caused by :: cant $subtract astring from a string, whereas I have added a conversion stage in the pipeline before the other stages, and I checked that mongodb pipelines are run sequentially, so the conversion should be taking place before the subtraction, which does not seem to be happening here. What could be going wrong?

Answer 1

You have to use
"convertedPageStart": { "$convert": { "input": "$page_start", ... } },
"convertedPageEnd": { "$convert": { "input": "$page_end", ... } }
"no_of_pages":{ "$subtract": ["$convertedPageEnd","$convertedPageStart"] }
i.e. you missed the $