MongoDB/WiredTiger - compress all collections in a database.

gistfile1.js

colls = db.getCollectionNames();
for (var i = 0; i < colls.length; i++) {
  var coll = colls[i];

  if (coll.match(/^system/)) continue;

  if (coll.match(/_old$/) || coll.match(/_comp$/)) {
    db[coll].drop();
    continue;
  }

  stats = db[coll].stats();
  if (stats.wiredTiger.creationString.match(/snappy/)) continue;

  db.createCollection(coll + "_comp", {storageEngine: {wiredTiger: {configString: 'block_compressor=snappy'}}});
  db[coll].find().forEach(function(o) {
    db[coll + "_comp"].save(o);
  });
  db[coll].renameCollection(coll + "_old");
  db[coll + "_comp"].renameCollection(coll);
}

Make sure you don't already have collections where name =~ /_old$/ || name =~ /_comp$/, or they will be deleted.