Logstash filter: copying values after split

I have azure event hub data coming into logstash via the eponymous plugin. I get huge junks of json that contain multiple events which I then have to split into distinct events. Here is a simplified example:

{"records": [
{ "LogicalServerName": "myservername", "SubscriptionId": "mysubid-uuid", "ResourceGroup": "Prod", "time": "2019-02-15T17:44:23.3870000Z", "resourceId": "/SUBSCRIPTIONS/mysubid-uuid/RESOURCEGROUPS/PROD/PROVIDERS/MICROSOFT.SQL/SERVERS/myservername/DATABASES/MASTER", "category": "SQLSecurityAuditEvents", "operationName": "AuditEvent"},
{ "LogicalServerName": "myservername", "SubscriptionId": "mysubid-uuid", "ResourceGroup": "Prod", "time": "2019-02-15T17:44:23.3870000Z", "resourceId": "/SUBSCRIPTIONS/mysubid-uuid/RESOURCEGROUPS/PROD/PROVIDERS/MICROSOFT.SQL/SERVERS/myservername/DATABASES/MASTER", "category": "SQLSecurityAuditEvents", "operationName": "AuditEvent"}

After the split I'm trying to copy the key LogicalServerName into a custom field called sourceHost. This is to normalize the input from a variety of services. Unfortunately, it doesn't seem to work. Records.LogicalServerName has the correct value, but the sourceHost field is just blank.

Here's the filter I'm trying to use:

filter {
    if [type] == "azure_event_hub" {
        json {
            source => "message"
        split {
            field => ["records"]
        mutate {
            # Map the requestHost to a standard hostname field
            copy => { "records.LogicalServerName" => "sourceHost" }

What am I missing here?


 copy => { "[records][LogicalServerName]" => "sourceHost" }

That was the magic! Thank you very much. I must not have been googling the right things, because that never came up as one of the dozen or so things I tried.

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