Currently we store the documents for both navigational directions of a bi-directional association in the same collection in MongoDB when using the COLLECTION_PER_ASSOCIATION strategy:

{
     "_id" : { "owners_id" : "owner0001" },
     "rows" : [
         "accountABC",
         "accountXYZ"
    ]
}
{
    "_id" : { "bankAccounts_id" : "accountABC" },
        "rows" : [
        "owner0001"
    ]
}
{
    "_id" : { "bankAccounts_id" : "accountXYZ" },
        "rows" : [
        "owner0001"
    ]
}

This requires to store the column name of the ids as we otherwise wouldn't know which side of the association a given record represents.

An alternative would be to use two separate collections for the both directions of the assocation, which allows for a more compact id syntax (assuming non-compound keys):

{
     "_id" : "owner0001",
     "rows" : [
         "accountABC",
         "accountXYZ"
    ]
}

{
    "_id" : "accountABC",
        "rows" : [
        "owner0001"
    ]
}
{
    "_id" : "accountXYZ",
        "rows" : [
        "owner0001"
    ]
}

In a way that would closely resemble the IN_ENTITY strategy, only that the assocations are not stored as sub-documents within the entities but as separate documents. It should also be a bit more efficient as the key is not structured and only the half of documents needs to be searched when accessing one side of an association.