You are absolutely right about the item_id possibly not being unique. It indeed was not as I was doing a SQL join that returned multiple times the same item with different LEFT JOIN columns values.
Secondly,the problem I had with the document_type and the index not getting assigned properly was so dumb also...
Basically I had it setup with capital letters and elastic seems to get angry at anything capitalized in the names of indexes and pretty much everything else :
doesn't work :
output {
elasticsearch {
hosts => "localhost:9200"
index => "%{Language}"
document_type => "%{MerchantID}"
document_id => "%{itemid}"
}
works :
output {
elasticsearch {
hosts => "localhost:9200"
index => "%{language}"
document_type => "%{merchantid}"
document_id => "%{itemid}"
}