A javascript can be provided within the couchdb settings that can munge the changes stream. The json provided to the script is under a var called ctx with the relevant seq stream change (for example, ctx.doc will refer to the document, or ctx.deleted is the flag if its deleted or not).

Note, this feature requires the lang-javascript plugin.

The ctx.doc can be changed and its value can will be indexed (assuming its not a deleted change). Also, if ctx.ignore is set to true, that change seq will be ignore and not applied.

Here is an example setting that adds field1 with value value1 to all docs.

{
    "type" : "couchdb",
    "couchdb" : {
        "script" : "ctx.doc.field1 = 'value1'"
    }
}