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Formatting with Show and List

See also the official documentation for the show and list topics.

Note that this is only available in CouchDB 0.9 or newer

The basics of formatting documents using show and list functions. These functions convert documents and views, respectively, into non-JSON formats. The rows of each view are processed individually, which keeps long lists from becoming memory hogs.

They are designed to be cacheable. CouchDB handles generating Etags for show and list responses.

Show and list functions are side effect free and idempotent. They can not make additional HTTP requests against CouchDB. Their purpose is to render JSON documents in other formats.

Showing Documents

Show functions are stored in your design document, under the shows key. Here's an example set of show functions:

   1 {
   2     "_id": "_design/examples",
   3     "shows": {
   4         "posts": "function(doc, req) { /*...*/ return responseObject;}",
   5         "people": "function(doc, req) { /*...*/ }"
   6     }
   7 }

Assuming these functions were in a design document named "examples" in a database named "db", they could be queried like this:

GET /db/_design/examples/_show/posts/somedocid

GET /db/_design/examples/_show/people/otherdocid

GET /db/_design/examples/_show/people/otherdocid?format=xml&details=true

The show function is run with two arguments. The first is the document corresponding to the requested docid, and the second describes the HTTP request's query string, Accept headers, and other per-request information. The function returns an object describing its HTTP response.

Example show function

   1 function(doc, req) {
   2   return {
   3     body: "Hello World"
   4   }
   5 }

If the show function is queried with document id that has no corresponding document in the database, doc is null and the submitted document shows up in req.id. This is useful for creating new documents with a name, like in a wiki.

If the show function is queried without a document id at all, doc is null and req.id is null. This is useful for creating new documents where the user specifies the new document id in a user interface, like in a CMS.

   1 function(doc, req) {
   2   if(doc) {
   3     // regular doc display logic
   4   } else { // document not found
   5     if(req.id) {
   6       // handle unused doc id
   7     } else {
   8       // handle unspecified doc id
   9     }
  10   }
  11 }

The request and response objects are of the same format used by _external functions, as documented in ExternalProcesses.

Since CouchDB 0.11.0 you can use the send() function as explained below in show functions as well.

Listing Views with CouchDB 0.10 and later

List functions are stored under the lists key of a design document. Here's an example design doc with list functions, in addition to views:

   1 {
   2     "_id": "_design/examples",
   3     "views" {
   4         "posts-by-date": {"map": "function(doc){ /*...*/ }"},
   5         "posts-by-tag": {"map": "function(doc){ /*...*/ }"},
   6         "people-by-name": {"map": "function(doc) { /*...*/ }"}
   7     },
   8     "lists": {
   9         "index-posts": "function(head, req) { /*...*/ }",
  10         "browse-people": "function(head, req) { /*...*/ }"
  11     }
  12 }

These lists are run by querying URLs like:

GET /db/_design/examples/_list/index-posts/posts-by-date?descending=true&limit=10

GET /db/_design/examples/_list/index-posts/posts-by-tag?key="howto"

GET /db/_design/examples/_list/browse-people/people-by-name?startkey=["a"]&limit=10

As above, we assume the database is named "db" and the design doc "examples".

Couchdb 0.10 supports an alternate form of URL which allows you to use a list function and a view from different design documents. This is particularly useful when you want to use a different language for the list and for the view. These URLs are very similar to the above examples, but instead of the tail portion being the name of the view, the tail portion can consist of two parts - a design doc name and the name of the view in that second document. For example:

GET /db/_design/examples/_list/index-posts/other_ddoc/posts-by-tag?key="howto"

[As above, we assume the database is named "db" and the design doc with the list is named "examples", while the design doc with the view is "other_ddoc".]

An Example list function

   1 function(head, req) {
   2   var row;
   3   start({
   4     "headers": {
   5       "Content-Type": "text/html"
   6      }
   7   });
   8   while(row = getRow()) {
   9     send(row.value);
  10   }
  11 }

Listing Views with CouchDB 0.9

List functions were introduced in CouchDB 0.9 and had different, more complex API. This API is no longer available after 0.10.

A list function has a more interesting signature, as it is passed the head of the view on first invocation, then each row in turn, then called one more time for the tail of the view. The function should check the head and row parameters to identify which state it's being called in; the sequence of calls to listfn, for a view with three rows, would look like:

   1 listfn(head, null,    req, null    );  // Before the first row: head is non-null
   2 listfn(null, rows[0], req, row_info);  // First row
   3 listfn(null, rows[1], req, row_info);  // Subsequent rows...
   4 listfn(null, rows[2], req, row_info);
   5 listfn(null, null,    req, row_info);  // After last row: row=null
   6 

The head parameter -- which is only passed into the first call -- contains an object with information about the view that is to be iterated over. It's much like the response object returned from a view query in the CouchDB JavaScript API; useful properties include total_rows and offset.

The row_info parameter contains an object with information about the iteration state. Its properties include:

Example list function:

   1 function(head, row, req, row_info) {
   2   if (head) {
   3     return "<p>head: "+JSON.stringify(head)+"</p><ul>";
   4   } else if (row) {
   5     return "<li>"+JSON.stringify(row)+"</li>";
   6   } else {
   7     return "</ul><h4>the tail</h4>"
   8   }
   9 }

Other Fun Things

Stopping iteration in a `_list`

If you want to terminate iteration of a _list early you can return a {stop: true} JSON object from any of the calls to the function that include a row object.

Sending a Redirect

In the call to _show or when _list is called with a head object you can control the headers and status code sent to the client. An example of this would be to send a redirect notification.

   1 function(doc) {
   2   return {"code": 302, "body": "See other", "headers": {"Location": "/"}};
   3 }

For CouchDB version 0.9:

   1 function(head, row, req, row_info) {
   2   if (head) {
   3     return {"code": 302, "body": "See other", "headers": {"Location": "/"}};
   4   } else if (row) {
   5     return {stop: true};
   6   } else {
   7     return "."
   8   }
   9 }

For CouchDB version 0.10:

   1 function(head, req) {
   2   start({"code": 302, "headers": {"Location": "/"}});
   3 }

See Advanced_Shows_and_Lists_Throwing_Redirects

Specifying Content-Type Response Header

There are two ways to deal with a content-type header in the response to a show or list request. The first way is to specify the content type as a member of the _show function's response object:

   1 return {
   2    "headers" : {"Content-Type" : "application/xml"},
   3    "body" : new XML('<xml><node foo="bar"/></xml>')
   4 }

Responding to different Content-Type Request Headers

The second way to deal with content-type headers is to rely on some global helper methods defined by CouchDB's <couchdb_install>/share/couchdb/server/main.js file. The registerType method lets you register a type key with one or more content-type strings. Please refer to the main.js file to see content-types registered by default.

   1 registerType("foo", "application/foo", "application/x-foo");

The other global helper method for handling varying Content-Type headers is respondWith. This helper method allows you to specify different response objects depending on the type key that corresponds to the content-type request header. The first argument is the request object, and the second argument is a key-value object that maps type keys to functions. Each function is expected to return an HTTP response object customized for the requested Content-Type.

   1 //... in your show function...
   2 return respondWith(req, {
   3          html : function() {
   4            return {
   5              body:"Ha ha, you said \"" + doc.word + "\"."
   6            };
   7          },
   8          foo : function() {
   9            return {
  10              body: "foofoo"
  11            };
  12          },
  13          fallback : "html"
  14        });

Since CouchDB 0.10.0 there is no responseWith-Method anymore. Please use the provides Method instead. For CouchDB version 0.10:

   1 //... in your show function...
   2 provides("html", function() {return "<b>doc.title</b>";});
   3 provides("xml", function() {return "<text class="title">doc.title</text>";});

Hopefully this is enough to get started. For a more complete set of examples, see the CouchDB test suite, especially show_documents.js and list_views.js

ETags

The ETag for your _show will always be the ETag of the underlying document. This means that if you sneakily try to provide dynamic content by doing things in your _show function, a browser will not refresh the page if the dynamic content changes; a browser will pass an If-None-Match header with the ETag from the previous time it called your _show, and CouchDB will return the same ETag (because the underlying document has not changed) even if the dynamic content you added has changed, and a 304 Not Modified header.

You can circumvent this. In CouchDB up to version 1.2, The things that go into calculating the ETag for a _show or _list URL are: the ETag of the underlying document, the requesting user-agent's Accept header, and the currently logged-in user's roles. So, if the thing you want to alter in the dynamic content is a "user is logged in"/"user is not logged in" header, make sure that your users have roles (it doesn't matter what they are); this will ensure that a logged-in user and a non-logged-in user will get different ETags for the same _show function on the same document.

In CouchDB 1.3, the Etag also uses the currently logged-in user's name; so different users (and logged-in user vs non-logged-in user) will get different ETags.

Shows without a document

Technically, you do not have to pass a docid at all. You can just

GET /db/_design/examples/_show/haha

to run the "haha" _show without any document at all. This can be a nice way of getting some server-side processing done; your "haha" show function might return the contents of a template, for example, and alter the content based on the current userCtx. Be sure and read the point above about ETags, though.

Formatting_with_Show_and_List (last edited 2013-06-05 21:57:52 by NathanVanderWilt)