filter_on_string_constant.ex

# elixir
{:ok, pid} = Mongo.start(database: "test")
coll = "tags"
pipeline = [
  %{'$project': %{ 'b': %{ '$in': ["b", ["a", "b", "c"]] } }},
  %{'$match': %{ 'b': true }},
  %{'$project': %{ 'b': 0 }}
]
Mongo.aggregate(pid, coll, pipeline) |> Enum.to_list

filter_on_string_constant.js

// in mongo cli
// Add some data
db.tags.insertMany([
  { ints: [1, 2, 3], name: "numbers" },
  { ints: [3, 4, 5], name: "next_numbers" },
  { ints: ["a", "b", "c"], name: "letters" },
])

// Select on the string constant
db.tags.aggregate([
  { $project: { "b": { $in: ["b", ["a", "b", "c"]] }}},
  { $match: { b: true }},
  { $project: { "b": 0 }}
]);