Thanks for the answer!
I tried running it in iex, and I seem to get the same problem here.
The query is successfully built but upon calling it I receive the following error:
[error] an exception was raised logging %DBConnection.LogEntry{call: :prepare_execute, connection_time: 40561270, decode_time: nil, params: [], pool_time: 242862, query: %Postgrex.Query{cache: :reference, columns: nil, name: "", param_formats: nil, param_oids: nil, param_types: nil, ref: nil, result_formats: nil, result_oids: nil, result_types: nil, statement: #Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>, types: nil}, result: {:error, %DBConnection.ConnectionError{message: "an exception was raised: ** (ArgumentError) argument error\n :erlang.iolist_size([\"\", 0, #Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>, 0, <<0, 0>>, \"\"])\n (postgrex) lib/postgrex/messages.ex:227: Postgrex.Messages.encode_msg/1\n (postgrex) lib/postgrex/protocol.ex:2774: anonymous fn/2 in Postgrex.Protocol.msg_send/3\n (elixir) lib/enum.ex:1948: Enum.\"-reduce/3-lists^foldl/2-0-\"/3\n (postgrex) lib/postgrex/protocol.ex:2774: Postgrex.Protocol.msg_send/3\n (postgrex) lib/postgrex/protocol.ex:1021: Postgrex.Protocol.parse_describe_flush/3\n (db_connection) lib/db_connection/holder.ex:277: DBConnection.Holder.holder_apply/4\n (db_connection) lib/db_connection.ex:1240: DBConnection.prepare/4\n (db_connection) lib/db_connection.ex:1233: DBConnection.run_prepare/4\n (db_connection) lib/db_connection.ex:1245: DBConnection.run_prepare_execute/5\n (db_connection) lib/db_connection.ex:1342: DBConnection.run/6\n (db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5\n (db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4\n (postgrex) lib/postgrex.ex:217: Postgrex.query_prepare_execute/4\n (ecto_sql) lib/ecto/adapters/sql.ex:292: Ecto.Adapters.SQL.query!/4\n (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6\n (elixir) src/elixir.erl:275: :elixir.eval_forms/4\n (iex) lib/iex/evaluator.ex:257: IEx.Evaluator.handle_eval/5\n (iex) lib/iex/evaluator.ex:237: IEx.Evaluator.do_eval/3\n (iex) lib/iex/evaluator.ex:215: IEx.Evaluator.eval/3\n"}}}: ** (ArgumentError) argument error
:erlang.iolist_to_binary(#Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>)
(postgrex) lib/postgrex/query.ex:99: String.Chars.Postgrex.Query.to_string/1
(ecto_sql) lib/ecto/adapters/sql.ex:772: Ecto.Adapters.SQL.log/4
(db_connection) lib/db_connection.ex:1377: DBConnection.log/5
(postgrex) lib/postgrex.ex:217: Postgrex.query_prepare_execute/4
(ecto_sql) lib/ecto/adapters/sql.ex:292: Ecto.Adapters.SQL.query!/4
(stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
(elixir) src/elixir.erl:275: :elixir.eval_forms/4
(iex) lib/iex/evaluator.ex:257: IEx.Evaluator.handle_eval/5
(iex) lib/iex/evaluator.ex:237: IEx.Evaluator.do_eval/3
(iex) lib/iex/evaluator.ex:215: IEx.Evaluator.eval/3
(iex) lib/iex/evaluator.ex:103: IEx.Evaluator.loop/1
(iex) lib/iex/evaluator.ex:27: IEx.Evaluator.init/4
(stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
** (ArgumentError) argument error
:erlang.iolist_size(["", 0, #Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>, 0, <<0, 0>>, ""])
(postgrex) lib/postgrex/messages.ex:227: Postgrex.Messages.encode_msg/1
(postgrex) lib/postgrex/protocol.ex:2774: anonymous fn/2 in Postgrex.Protocol.msg_send/3
(elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
(postgrex) lib/postgrex/protocol.ex:2774: Postgrex.Protocol.msg_send/3
(postgrex) lib/postgrex/protocol.ex:1021: Postgrex.Protocol.parse_describe_flush/3
(db_connection) lib/db_connection/holder.ex:277: DBConnection.Holder.holder_apply/4
(db_connection) lib/db_connection.ex:1240: DBConnection.prepare/4
(db_connection) lib/db_connection.ex:1233: DBConnection.run_prepare/4
(db_connection) lib/db_connection.ex:1245: DBConnection.run_prepare_execute/5
(db_connection) lib/db_connection.ex:1342: DBConnection.run/6
(db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5
(db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4
(postgrex) lib/postgrex.ex:217: Postgrex.query_prepare_execute/4
(ecto_sql) lib/ecto/adapters/sql.ex:292: Ecto.Adapters.SQL.query!/4
iex(10)> [error] GenServer #PID<0.370.0> terminating
** (DBConnection.ConnectionError) client #PID<0.547.0> stopped: ** (ArgumentError) argument error
:erlang.iolist_size(["", 0, #Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>, 0, <<0, 0>>, ""])
(postgrex) lib/postgrex/messages.ex:227: Postgrex.Messages.encode_msg/1
(postgrex) lib/postgrex/protocol.ex:2774: anonymous fn/2 in Postgrex.Protocol.msg_send/3
(elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
(postgrex) lib/postgrex/protocol.ex:2774: Postgrex.Protocol.msg_send/3
(postgrex) lib/postgrex/protocol.ex:1021: Postgrex.Protocol.parse_describe_flush/3
(db_connection) lib/db_connection/holder.ex:277: DBConnection.Holder.holder_apply/4
(db_connection) lib/db_connection.ex:1240: DBConnection.prepare/4
(db_connection) lib/db_connection.ex:1233: DBConnection.run_prepare/4
(db_connection) lib/db_connection.ex:1245: DBConnection.run_prepare_execute/5
(db_connection) lib/db_connection.ex:1342: DBConnection.run/6
(db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5
(db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4
(postgrex) lib/postgrex.ex:217: Postgrex.query_prepare_execute/4
(ecto_sql) lib/ecto/adapters/sql.ex:292: Ecto.Adapters.SQL.query!/4
(stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
(elixir) src/elixir.erl:275: :elixir.eval_forms/4
(iex) lib/iex/evaluator.ex:257: IEx.Evaluator.handle_eval/5
(iex) lib/iex/evaluator.ex:237: IEx.Evaluator.do_eval/3
(iex) lib/iex/evaluator.ex:215: IEx.Evaluator.eval/3
(db_connection) lib/db_connection/connection.ex:162: DBConnection.Connection.handle_cast/2
(connection) lib/connection.ex:810: Connection.handle_async/3
(stdlib) gen_server.erl:637: :gen_server.try_dispatch/4
(stdlib) gen_server.erl:711: :gen_server.handle_msg/6
(stdlib) proc_lib.erl:259: :proc_lib.wake_up/3
Last message: {:"$gen_cast", {:stop, #Reference<0.3943072288.1394475010.34145>, %DBConnection.ConnectionError{message: "client #PID<0.547.0> stopped: ** (ArgumentError) argument error\n :erlang.iolist_size([\"\", 0, #Ecto.Query<from p0 in Sombra.News.Post, where: p0.published? == true, select: p0>, 0, <<0, 0>>, \"\"])\n (postgrex) lib/postgrex/messages.ex:227: Postgrex.Messages.encode_msg/1\n (postgrex) lib/postgrex/protocol.ex:2774: anonymous fn/2 in Postgrex.Protocol.msg_send/3\n (elixir) lib/enum.ex:1948: Enum.\"-reduce/3-lists^foldl/2-0-\"/3\n (postgrex) lib/postgrex/protocol.ex:2774: Postgrex.Protocol.msg_send/3\n (postgrex) lib/postgrex/protocol.ex:1021: Postgrex.Protocol.parse_describe_flush/3\n (db_connection) lib/db_connection/holder.ex:277: DBConnection.Holder.holder_apply/4\n (db_connection) lib/db_connection.ex:1240: DBConnection.prepare/4\n (db_connection) lib/db_connection.ex:1233: DBConnection.run_prepare/4\n (db_connection) lib/db_connection.ex:1245: DBConnection.run_prepare_execute/5\n (db_connection) lib/db_connection.ex:1342: DBConnection.run/6\n (db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5\n (db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4\n (postgrex) lib/postgrex.ex:217: Postgrex.query_prepare_execute/4\n (ecto_sql) lib/ecto/adapters/sql.ex:292: Ecto.Adapters.SQL.query!/4\n (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6\n (elixir) src/elixir.erl:275: :elixir.eval_forms/4\n (iex) lib/iex/evaluator.ex:257: IEx.Evaluator.handle_eval/5\n (iex) lib/iex/evaluator.ex:237: IEx.Evaluator.do_eval/3\n (iex) lib/iex/evaluator.ex:215: IEx.Evaluator.eval/3\n"}, %Postgrex.Protocol{buffer: "", connection_id: 13921, connection_key: 726524142, disconnect_on_error_codes: [], null: nil, parameters: #Reference<0.3943072288.1394343937.29568>, peer: {{127, 0, 0, 1}, 5432}, postgres: :idle, queries: #Reference<0.3943072288.1394475009.29554>, sock: {:gen_tcp, #Port<0.12>}, timeout: 15000, transactions: :naive, types: {Postgrex.DefaultTypes, #Reference<0.3943072288.1394475012.29604>}}}}
State: {Postgrex.Protocol, %Postgrex.Protocol{buffer: "", connection_id: 13921, connection_key: 726524142, disconnect_on_error_codes: [], null: nil, parameters: #Reference<0.3943072288.1394343937.29568>, peer: {{127, 0, 0, 1}, 5432}, postgres: :idle, queries: #Reference<0.3943072288.1394475009.29554>, sock: {:gen_tcp, #Port<0.12>}, timeout: 15000, transactions: :naive, types: {Postgrex.DefaultTypes, #Reference<0.3943072288.1394475012.29604>}}}