can't update a customer with put in erlang - post

Question

I'm trying to update a customer in my riak database and i get the error message below:
And I don't know what causing this error and what this error message means.
And the module I use is:
allowed_methods(Request, State) ->
{['PUT'], Request, State}.
content_types_accepted(Request, State) ->
{[{"application/json",to_json}], Request, State}.
The error
webmachine error: path="/customer/cus/update" {error, {error,undef, [{customer_update,to_json, [{wm_reqdata,'PUT',http,
{1,1}, "127.0.0.1", {wm_reqstate,#Port<0.6513>, {dict,4,16,16,8,80,48,
{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}, {{[],[],[],
[[mediaparams,{"charset","UTF-8"}]], [],
[[resource_module|customer_update],
['content-type',116,101,120,116,47,104,116,109, 108]], [],
[['content-encoding',105,100,101,110,116,105,116, 121]],
[],[],[],[],[],[],[],[]}}},
undefined,"127.0.0.1",'REQDATA',undefined,undefined,
{wm_log_data,undefined, {1322,989559,450145}, 'PUT', {6,
{"content-length", {'Content-Length',"121"},
{"connection",{'Connection',"Keep-Alive"},nil,nil}, {"content-type",
{'Content-Type', "application/json; charset=UTF-8"}, nil, {"host",
{'Host',"localhost:8000"},
{"expect",{"Expect","100-Continue"},nil,nil}, {"user-agent",
{'User-Agent', "Apache-HttpClient/4.0.1 (java 1.5)"}, nil,nil}}}}},
"127.0.0.1","/updatecustomer", {1,1},
404,0,undefined,undefined,undefined}},
[],"/customer/cus/update","//customer/cus/update",
{dict,0,16,16,8,80,48,
{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
{{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}}},
[],".",500,1073741824,67108864,[],[], {6, {"content-length",
{'Content-Length',"121"},
{"connection",{'Connection',"Keep-Alive"},nil,nil}, {"content-type",
{'Content-Type',"application/json; charset=UTF-8"}, nil, {"host",
{'Host',"localhost:8000"},
{"expect",{"Expect","100-Continue"},nil,nil}, {"user-agent",
{'User-Agent',"Apache-HttpClient/4.0.1 (java 1.5)"}, nil,nil}}}}},
not_fetched_yet,false,
{1,{"content-type",{"Content-Type","text/html"},nil,nil}}, <<>>,
["localhost"], 8000,[]}, undefined]},
{webmachine_resource,resource_call,3}, {webmachine_resource,do,3},
{webmachine_decision_core,resource_call,1},
{webmachine_decision_core,accept_helper,0},
{webmachine_decision_core,decision,1},
{webmachine_decision_core,handle_request,2},
{webmachine_mochiweb,loop,1}]}}

Answer 1

You should define to_json/2 function.
For example:
to_json(RD, Result) ->
{mochijson:encode(Result), RD, Result}.

Answer 2

Unfortunately I lack the reputation to comment on the answer by Ilya.
TLDR: prefix to_json with the name of the module where you defined it
Longer answer:
I'm defining to_json in another module
Looking at your content_types_accepted/2 call, you aren't specifying which module to_json resides in, hence the undef error. Erlang function calls are always MFA -> module:function(arguments), you can only omit the module if the function is in the same module.
See also the documentation on Erlang packages

Answer 3

The key to understanding this error is the part:
{error, {error,undef, [{customer_update,to_json, ...
Which reports an undef error. Errors of these kinds are described at:
http://www.erlang.org/doc/reference_manual/errors.html#id81244
And you can see that undef means we have an undefined function. The error is due to a call customer_update:to_json(..) which then did not exist. That is the problem you have here.