similar to a capture group but for a matched subdocument

In the database, there is a Chats collection with a participants subdocument structured as follows:

{ 
    _id: '1',
    participants: [ { _id: 'A', seen: false },  { _id: 'B', seen: false } ],
    messages: []
}

Each chat consists of only 2 participants, and one of them is always the currentUser, although which one is unknown. Additionally, each pair of users has only one chat associated with it.

When searching for a chat, both user ids are required. The query to find a chat looks like this:

Chats.find( 
    {   $and: [ 
        { participants: {$elemMatch: {_id: otherUserId}}}, 
        { participants: {$elemMatch: {_id: currentUserId}}} 
    ]}
)

The goal is to allow the currentUser to update their own seen field in a single operation.

Currently, the process involves finding the chat first, determining which participant represents the currentUser, creating a document to update that participant, and then updating it separately.

Is there a way to capture the id of the element matched with currentUserId, similar to using regex capture groups? For example...

Chats.update(   
    {   $and: [ 
        { participants: {$elemMatch: {_id: otherUserId}}}, 
        { participants: capture({$elemMatch: {_id: currentUserId}})} 
    ]},  
    {   
        $set: {"participants.(capture[0]).seen": true}
    })

Alternatively, is there a more efficient approach to achieve this?

javascriptmongodbmeteor

Answer №1

Although it might not be the exact solution you're seeking, I wanted to mention it just in case it proves helpful.

Chats.update(   {   $and: [ 
    { participants: {$elemMatch: {_id: otherUserId}}}, 
    { participants: {$elemMatch: {_id: currentUser}}} 
]},  
{   
    $set: {"participants.$.seen": true}
})

This approach should work for your situation because the $elemMatch function stores the index of the matched array. By carrying out an and operation with $eleMatch, it will save the index matched by the last $eleMatch, which is typically the current user. Using the positional operator in this context will then update the seen field for the current user accordingly.

Answer №2

To efficiently update the 'seen' status, consider structuring the data in this way:

{
    _id: '1',
    participants: ['C', 'D'],
    seen: [],
    messages: []

}

Then, you can mark user 'C' as having seen the message by using this code snippet:

Chats.update(
  { $and: [ {participants: 'C'}, {participants: 'D'} ] },
  { $addToSet: { seen: 'C' } }
)
  • $elemMatch is not needed for a single query condition.
  • $addToSet will only include 'C' if it is not already present.

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