Cannot bind variables within on clause of query joins

[stavro]

Environment

• Elixir version: 1.3.0
• Database and version: Postgres 9.5
• Ecto version: 2.0.5
• Database adapter and version: Postgrex v0.12
• Operating system: OS X

Current behavior

Variables cannot be bound in the on clause of an Ecto.Query join:

one = 1

Repo.all(
  from u in User,
  left_join: staff in assoc(u, :staff), on: 1 == ^one,
  select: u.id
)

Error:

** (ArgumentError) parameters must be of length 1 for query %Postgrex.Query{columns: ["id"], copy_data: false, decoders: [#Function<8.109917431/1 in Postgrex.Types.decoder/2>], encoders: [#Function<9.109917431/1 in Postgrex.Types.encoder/2>], name: "ecto_31925", null: nil, param_formats: [:binary], param_info: [{23, %Postgrex.TypeInfo{array_elem: 0, base_type: 0, comp_elems: [], input: "int4in", oid: 23, output: "int4out", receive: "int4recv", send: "int4send", type: "int4"}, Postgrex.Extensions.Int4}], ref: #Reference<0.0.5.5707>, result_formats: [:binary], result_info: [{2950, %Postgrex.TypeInfo{array_elem: 0, base_type: 0, comp_elems: [], input: "uuid_in", oid: 2950, output: "uuid_out", receive: "uuid_recv", send: "uuid_send", type: "uuid"}, Postgrex.Extensions.UUID}], statement: "SELECT u0.\"id\" FROM \"users\" AS u0 LEFT OUTER JOIN \"staff\" AS s1 ON (s1.\"user_id\" = u0.\"id\") AND (1 = $1)", types: 14135761}
[debug] QUERY ERROR source="users" db=2.0ms
SELECT u0."id" FROM "users" AS u0 LEFT OUTER JOIN "staff" AS s1 ON (s1."user_id" = u0."id") AND (1 = $1) []
         (postgrex) lib/postgrex/query.ex:78: DBConnection.Query.Postgrex.Query.encode/3
    (db_connection) lib/db_connection.ex:859: DBConnection.describe_execute/4
    (db_connection) lib/db_connection.ex:930: anonymous fn/4 in DBConnection.run_meter/5
    (db_connection) lib/db_connection.ex:987: DBConnection.run_begin/3
    (db_connection) lib/db_connection.ex:463: DBConnection.prepare_execute/4
             (ecto) lib/ecto/adapters/postgres/connection.ex:78: Ecto.Adapters.Postgres.Connection.prepare_execute/5
             (ecto) lib/ecto/adapters/sql.ex:235: Ecto.Adapters.SQL.sql_call/6
             (ecto) lib/ecto/adapters/sql.ex:424: Ecto.Adapters.SQL.execute_and_cache/7
             (ecto) lib/ecto/repo/queryable.ex:127: Ecto.Repo.Queryable.execute/5
             (ecto) lib/ecto/repo/queryable.ex:40: Ecto.Repo.Queryable.all/4

Expected behavior

A successful query (:happy panda:), or a better error message (:sad panda:).

I know we can use fragments, but this current error is confusing and unexpected. Would love to see this improved!

[stavro]

You're amazing.

❤️