Dynamic query fields in Sor / Macro substitution in solrconfig.xml - solr

Question

We've got a multilingual search index with the "field-per-language" configuration with a lot of similar aliases in the search handler like this:
<str name="f.content_en.qf">Title_en^10 Text_en^1 ...</str>
<str name="f.content_de.qf">Title_de^10 Text_de^1 ...</str>
...
They are used in the q parameter:
<str name="q">{!edismax qf=$searchField pf=$searchField v=$searchText}</str>
The client knows, which language should be used and calls Solr like this, e.g.: /solr/core/search?searchText=TEXT&searchField=content_en
That works fine, but the configuration contains a lot of similar stuff.
I'd like to optimize the config to something like this:
<str name="df">content</str>
<str name="f.content.qf">Title_${lang}^10 Text_${lang}^1...</str>
Then the client would need to provide the lang parameter only.
I tried to use concat function like this:
paramLang=en
searchFields=concat("Title", "_", "${paramLang}", " ", "Text", "_", "${paramLang}")
and use it as the qf:
q={!edismax qf=$searchFields v=$searchText}
But it seems, the local params qf does not support Solr functions.
Is is possible with Solr at all?

Answer 1

Actually, the Parameter substitution / Macro Expansion works fine.
The issue was with those macros in the solrconfig.xml: there is a conflict with Solr system properties substitution. Solr could not resolve the query parameter macros.
I could not find a proper way, how to escape query parameters (macros) and used the following workaround:
<lst name="invariants">
<str name="defType">edismax</str>
<str name="searchFields">
Title_${lang:${lang}}^10
Text_${lang:${lang}}^1
...
<lst name="defaults">
<str name="q">*</str>
<str name="qf">${searchFields:${searchFields}}</str>
<str name="pf">${searchFields:${searchFields}}</str>
<str name="lang">en</str>
...
Query URL: /search?q=TEXT&lang=en
Update: proper way to deal with var substitution in solrconfig.xml - escape the dollar char by $$:
<str name="searchFields">
Title_$${lang}^10
Text_$${lang}^1
...
Update #2: do NOT define macros in the invariant or append sections when using a Solr Cloud! Otherwise, you'll a weird exception, e.g.:
"undefined field: \"Text_$\"
or
"msg": "Error from server at null: org.apache.solr.search.SyntaxError: Query Field '${searchFields}' is not a valid field name"
P.S. wt=json as "invariant" is also NOT compatible with Solr Cloud, giving "unexpected" content-type error.
So many "surprises" :(