Concatenate two fields

[Jasmin]

hi ,
i am using mutate add_field to concatenate two fileds to make them a unique id ,
but ouptut give me this :
{
"_index" : "sql",
"_type" : "_doc",
"_id" : " %{GL_ETABLISSEMENT} %{GL_NUMERO} ",
"_score" : 1.0,
"_source" : {
"gp_numero" : 205029243,
"gl_qtefact" : 1.0,
"gl_datepiece" : "2019-05-11T23:00:00.000Z",
"gl_etablissement" : "205",
"gl_article" : "28ZFPNG01 00502N X",
"gl_totalttc" : 41.757,
"gl_representant" : "101810",
"gp_dtsvlastchange" : "2019-05-12T14:17:41.147Z",
"gp_dtsvcreate" : "2019-05-12T14:17:41.147Z",
"gl_puttcbase" : 44.9,
"gl_codearticle" : "28ZFPNG01",
"@timestamp" : "2020-07-19T21:33:51.173Z",
"@version" : "1",
"newfield" : " %{GL_ETABLISSEMENT} %{GL_NUMERO}"
}

I CANNOT concatenate .

[Badger]

Field name are case sensitive. Try

mutate { add_field => { "newfield" => "%{gl_etablissement}" %{gp_numero" } }

It's gp_numero, not gl_numero, right?