r json mongodb query $in operator syntax error due to double quotes?

I'm building a json query to pass to a mongodb database in R.

In one scenario, I have a vector of dates and I want to query the database to return all records which have a date in the relevant field that matches a date in my vector of dates.

The second scenario is the same as the first, but this time I have a vector of character strings (IDs) and need to return all the records with matching IDs.

I understood the correct way to do this in a json query is to use the $in operator, and then put my vector in an array.

However, when I pass the query to my mongodb database, the exportLogId returns NULL. I'm quite sure that the problem is something to do with how I am representing the $in operator in the final query, since I have very similarly structured queries without the $in operator and they are all working. If I look for just one of my target dates or character strings, I get the desired result.

I followed the mongodb manual here to construct my query, and the only issue I can see is that the $in operator in the output of jsonlite::toJSON() is enclosed in double quotes; whereas I think it might need to be in single quotes (or no quotes at all, but I don't know how to write the syntax for that).

I'm creating my query in two steps:

1. Create the query as a series of nested lists
2. Convert the list object to json with jsonlite::toJSON()

Here is my code:

# Load libraries:
library(jsonlite)

# Create list of example dates to query in mongodb format:
sampledates <- c("2022-08-11T00:00:00.000Z",
                 "2022-08-15T00:00:00.000Z", 
                 "2022-08-16T00:00:00.000Z",
                 "2022-08-17T00:00:00.000Z", 
                 "2022-08-19T00:00:00.000Z")

# Create query as a list object:
    query_list_l <- list(filter =

                         # Add where clause:
                         list(where =

                                # Filter results by list of sample dates:
                                list(dateSampleTaken = list('$in' = sampledates),

                                     # Define format of column names and values:
                                     useDbColumns = "true",
                                     dontTranslateValues = "true",
                                     jsonReplaceUndefinedWithNull = "true"),

                              # Define columns to return:
                              fields = c("id",                
                                         "updatedAt",         
                                         "person.visualId",   
                                         "labName",           
                                         "sampleIdentifier",  
                                         "dateSampleTaken",   
                                         "sequence.hasSequence")))

# Convert list object to JSON:
query_json = jsonlite::toJSON(x = query_list_l, 
                              pretty = TRUE, 
                              auto_unbox = TRUE)

The JSON query now looks like this:

> query_json
{
  "filter": {
    "where": {
      "dateSampleTaken": {
        "$in": ["2022-08-11T00:00:00.000Z", "2022-08-15T00:00:00.000Z", "2022-08-16T00:00:00.000Z", "2022-08-17T00:00:00.000Z", "2022-08-19T00:00:00.000Z"]
      },
      "useDbColumns": "true",
      "dontTranslateValues": "true",
      "jsonReplaceUndefinedWithNull": "true"
    },
    "fields": ["id", "updatedAt", "person.visualId", "labName", "sampleIdentifier", "dateSampleTaken", "sequence.hasSequence"]
  }
} 

As you can see, $in is now enclosed in double quotes, even though I put it in single quotes when I created the query as a list object. I have tried replacing with sprintf() but that just adds a lot of backslashes to my query. I also tried:

query_fixed <- gsub(pattern = "\\"\\$\\in\\"", 
                    replacement = "\\'$in\\'", 
                    x = query_json)

... but this fails with an error.

I would be very grateful to know if:

1. The syntax problem that is preventing $in from working is actually the double quotes?
2. If double quotes is the problem, how do I replace them with single quotes without messing up the JSON format?

UPDATE:

The issue seems to occur when R is passing the query to the database, but I still can't work out exactly why.

If I try the query out in loopback explorer in the database, it works and using the export log ID produced, I can then fetch the results with httr::GET() in R. Example query results are shown below (sorry for the hashes - the main point is you can see the format of the returned values):

[1] "[{\"_id\":\"e59953b6-a106-4b69-9e25-1c54eef5264a\",\"updatedAt\":\"2022-09-12T20:08:39.554Z\",\"dateSampleTaken\":\"2022-08-16T00:00:00.000Z\",\"labName\":\"LNG_REFERENCE_DATA_CATEGORY_LAB_NAME_LAB_A\",\"sampleIdentifier\":\"LS0044-SCV2-PCR\",\"sequence\":{\"hasSequence\":false},\"person\":{\"visualId\":\"C-2022-0002\"}},{\"_id\":\"af5cd9cc-4813-4194-b60b-7d130bae47bc\",\"updatedAt\":\"2022-09-12T20:11:07.467Z\",\"dateSampleTaken\":\"2022-08-17T00:00:00.000Z\",\"labName\":\"LNG_REFERENCE_DATA_CATEGORY_LAB_NAME_LAB_A\",\"sampleIdentifier\":\"LS0061-SCV2-PCR\",\"sequence\":{\"hasSequence\":false},\"person\":{\"visualId\":\"C-2022-0003\"}},{\"_id\":\"b5930079-8d57-43a8-85c0-c95f7e0338d9\",\"updatedAt\":\"2022-09-12T20:13:54.378Z\",\"dateSampleTaken\":\"2022-08-16T00:00:00.000Z\",\"labName\":\"LNG_REFERENCE_DATA_CATEGORY_LAB_NAME_LAB_A\",\"sampleIdentifier\":\"LS0043-SCV2-PCR\",\"sequence\":{\"hasSequence\":false},\"person\":{\"visualId\":\"C-2022-0004\"}}]"