This is a follow on from http://en.wikibooks.org/wiki/XQuery/Freebase
which originated from a problem Michael Westbay assisted me with. Again it illustrates how to obtain information from Freebase via it's MQL language (it predated Sparql). The previous query was taken from https://developers.google.com/freebase/v1/mql-overview and it limits the data that results from the call to the Freebase API. You can see a limit parameter being set to 3 below in the API call. https://www.googleapis.com/freebase/v1/mqlread?query=[{"type":"/music/album","name":null,"artist":{"id":"/en/bob_dylan"},"limit":3}]&cursor If you do not specify a limit with your API call, Freebase will impose a limit of 100 records on your query. This message addresses the question of how to get everything. The key to doing this is dangling at the end of the above API call, it's the cursor parameter and it's usage is discussed with an example here https://developers.google.com/freebase/v1/mql-overview#querying-with-cursor-paging-results To summarise you ask for a cursor (see the example API call above for the form of the initial request) to be returned with your query results which acts as a link to the next set of query results. You obtain that next set by supplying the value of the cursor returned from the previous invocation. Along with that next set you get another cursor that points to the set after that. When the final set of results are retrieved the cursor returns a string value of false (the Freebase overview has this in upper case but my code used lower case 'false' and that works). The overview has sample Python code which I have not tried or parsed in anger but which I believe invokes libraries that take care of all the cursor handling for you. https://developers.google.com/freebase/v1/mql-overview#looping-through-cursor-results However the same thing can easily be achieved from XQuery with a little bit of tail recursion. We will use as an example a MQL query that returns all films with their netflix_id's. [{ "type": "/film/film", "name": null, "netflix_id": [] }] A few brief comments about MQL. You ask for something by giving the field name and a value null. Null gets replaced by the actual value. However if the field can have multiple values MQL will return an array and cause your null query to error. This may happen even when you are expecting a singular value so you can avoid this problem by using the symbol for an empty array instead of null as in the query above. You can paste the query above into http://www.freebase.com/query to see the results (we will take care of the cursor in the code example). Now to the code, which assumes XQuery 3.0 xquery version "3.0"; import module namespace xqjson="http://xqilla.sourceforge.net/lib/xqjson";; Freebase returns JSON but we want to store this in an xml db so we use the above package for json to XML conversion. From eXist you can install the package by just clicking it on the eXist Package Manager which you can get to from the eXist Dashboard. We declare a variable for our query. declare variable $mqlQuery {'[{ "type": "/film/film", "name": null, "netflix_id": [] }]'}; declare variable $freebase {'https://www.googleapis.com/freebase/v1/mqlread'}; declare variable $key {obtain an API key from freebase and puts it's value here'}; Here is a link to a blog that describes the process of obtaining a Freebase API key http://anchetawern.github.io/blog/2013/02/11/getting-started-with-freebase-api/ Since we are going to be doing tail recursion we need to put the API call in a function. Lets start with the function signature. declare function local:freebaseCall($cursor as xs:string,$i as xs:integer) 2 parameters the first is the cursor and the second an integer which I use to provision an auto-incremented unique file name an to tell me how many records were loaded at the end - since there are a 100 records per API call it's ( $i - 1) * 100 + the number of records returned by the final cursor. This function will make the API call and store the results in the db { if ($cursor eq 'false') termination condition then $i || ' pages loaded' else let $params := ('query=' || encode-for-uri($mqlQuery), 'key=' || $key, 'cursor=' || encode-for-uri($cursor)) Above uri encodes the parameters to the API call - we have three the MQL query, the API key and the cursor let $href := $freebase || '?' || string-join($params, '&') This constructs the API call - again thanks to Michael Westbay for showing the correct way to do this by string joining the parameters with a separator of & let $responses := http:send-request(<http:request href="{$href}" method="get"/>) Make the API call. return if ($responses[1]/@status ne '200') then <failure href="{xmldb:decode-uri(xs:anyURI($href))}">{$responses[1]}</failure> else let $jsonResponse:= util:base64-decode($responses[2]) Standard EXPATH http error checking - don't forget to base64 decode the body of the response. let $freebaseXML:= xqjson:parse-json($jsonResponse) Convert the returned JSON to XML because we are going to construct an http PUT to store it in our xml db. let $movieData := http:send-request(<http:request href="{concat(path to store the data in your repostiory,$i,'.xml')}" username="username" password="password" auth-method="basic" send-authorization="true" method="put"> <http:body media-type="application/xml"/> </http:request>, (), <batch cursor="{$cursor}">{transform:transform($freebaseXML,doc(identity.xsl'),())}</batch>) Standard EXPATH PUT request. On the last line we are wrapping the returned XML with an element that carries the value of the cursor that was used to obtain the page. Identity.xsl is of course the standard XSLT identity transform, you can use it as a placeholder for the insertion of your own custom transform. return local:freebaseCall($freebaseXML//data(pair[@name="cursor"]), $i + 1) Finally the tail recursive call. We extract the cursor from the returned JSON for parameter 1 and increment $i to give us a unique document name for the next page to store. }; Don't forget your closing curly brace and the attendant semi colon One last thing to kick it all off pass the null string as the initial cursor value and initialise your counter local:freebaseCall('',1) One last thing. I experienced repeated eXist crashes when running this. Note that you can prime the function call with a cursor to pick up from where you left off. That way you'll get to the end. return local:freebaseCall($freebaseXML//data(pair[@name="cursor"]), $i + 1) }; _______________________________________________ [email protected] http://x-query.com/mailman/listinfo/talk
