Find MongoDB records where array field is empty - mongodb

Question

I need to output a list of sketchid values for which tdata birthdate is not present/null, considering only the first birthdate in each tdata.
There are many fields in each collection, but I am only interested in two of them:
{
"sketchid": NumberLong(51148),
"tdata": "[
{ "birthdate": ["529718400"], ...},
{ "birthdate": null, ... }
]
}
I need to print the sketchid for each tdata in which the first birthdate is empty.
Note: A single tdata contains many birthdate fields, I need to check only the first birthdate field and if it is empty I need to output the sketchid for that set.
I have tried this, but I couldn't get the solution.
db.timeline.find({tdata:{birthdate:{$exists: true, “$ne”: null }}},{sketchid})
To illustrate the data schema, the find() command (db.timeline.find().pretty()) gives the following output:
{
"_id" : ObjectId("56ea7b9490e2a5b17c143b56"),
"sketchid" : NumberLong(51147),
"tdata" : "[{\\\"type\\\":\\\"dob\\\",\\\"title\\\":\\\"Vishakha Singh\\\",\\\"year\\\":null,\\\"color\\\":null,\\\"content\\\":\\\"\\\",\\\"birthdate\\\":[\\\"831254400\\\"],\\\"birthplace\\\":[\\\"Abu Dhabi, UAE\\\"],\\\"zodiac\\\":\\\"Taurus\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Cameo Role\\\",\\\"year\\\":[\\\"2014\\\"],\\\"color\\\":[\\\"#85330B\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Tamil movie Oru Oorla Rendu Raja&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Tamil movie Oru Oorla Rendu Raja<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Noticable Flick\\\",\\\"year\\\":[\\\"2013\\\"],\\\"color\\\":[\\\"#21C8F5\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Neetu in Fukrey&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Neetu in Fukrey<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Gained Recognition\\\",\\\"year\\\":[\\\"2010\\\"],\\\"color\\\":[\\\"#C90688\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Ashutosh Gowarikar\\'s Khelein Hum Jee Jaan Sey&quot;]" data-sheets-userformat="[null,null,47617,[null,0],null,null,null,null,null,null,null,null,0,null,[null,2,721024],&quot;sans-serif&quot;,11,null,1]">Ashutosh Gowarikar&#8217;s Khelein Hum Jee Jaan Sey<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Movie Debut\\\",\\\"year\\\":[\\\"2008\\\"],\\\"color\\\":[\\\"#FFCF00\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Hum Se Jahan&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Hum Se Jahan<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"quote-card\\\",\\\"title\\\":\\\"vishakha singh\\\",\\\"year\\\":null,\\\"color\\\":null,\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;I am not really sophisticated but happy and chilled out&quot;]" data-sheets-userformat="[null,null,14915,[null,0],[null,2,16777215],null,null,null,null,0,null,null,0,null,[null,2,0],&quot;helvetica&quot;,11]">I am not really sophisticated but happy and chilled out<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"}]"
}
{
"_id" : ObjectId("56ea7b9490e2a5b17c143b57"),
"sketchid" : NumberLong(51148),
"tdata" : "[{\\\"type\\\":\\\"dob\\\",\\\"title\\\":\\\"Ali Fazal\\\",\\\"year\\\":null,\\\"color\\\":null,\\\"content\\\":\\\"\\\",\\\"birthdate\\\":[\\\"529718400\\\"],\\\"birthplace\\\":[\\\"Lucknow, Uttar Pradesh\\\"],\\\"zodiac\\\":\\\"Libra\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Bollywood Appearance\\\",\\\"year\\\":[\\\"2016\\\"],\\\"color\\\":[\\\"#C90688\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Anand L. Rai\\'s Happy Bhaag Jayegi&quot;]" data-sheets-userformat="[null,null,47617,[null,0],null,null,null,null,null,null,null,null,0,null,[null,2,721024],&quot;sans-serif&quot;,11,null,1]">Anand L. Rai&#8217;s Happy Bhaag Jayegi<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"English Movie\\\",\\\"year\\\":[\\\"2015\\\"],\\\"color\\\":[\\\"#FFCF00\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Special Appearance in Furious 7&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Special Appearance in Furious 7<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"First Lead Role\\\",\\\"year\\\":[\\\"2015\\\"],\\\"color\\\":[\\\"#B6D402\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Khamoshiyan&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Khamoshiyan<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Supporting Role\\\",\\\"year\\\":[\\\"2014\\\"],\\\"color\\\":[\\\"#FF2305\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Bobby Jasoos with Vidya Balan&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Bobby Jasoos with Vidya Balan<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"First Success\\\",\\\"year\\\":[\\\"2013\\\"],\\\"color\\\":[\\\"#85330B\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Fukrey&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Fukrey<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Bollywood Debut\\\",\\\"year\\\":[\\\"2009\\\"],\\\"color\\\":[\\\"#21C8F5\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Special Appearance in 3 Idiots&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Special Appearance in 3 Idiots<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"American Miniseries\\\",\\\"year\\\":[\\\"2009\\\"],\\\"color\\\":[\\\"#C90688\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Bollywood Hero&quot;]" data-sheets-userformat="[null,null,513,[null,0],null,null,null,null,null,null,null,null,0]">Bollywood Hero<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"timeline\\\",\\\"title\\\":\\\"Screen Debut\\\",\\\"year\\\":[\\\"2008\\\"],\\\"color\\\":[\\\"#FFCF00\\\"],\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;Small role in the English language film The Other End of the Line&quot;]" data-sheets-userformat="[null,null,309761,[null,0],null,null,null,null,null,null,null,null,0,null,[null,2,721024],&quot;sans-serif&quot;,11,null,1,null,null,1]">Small role in the English language film The Other End of the Line<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"},{\\\"type\\\":\\\"quote-card\\\",\\\"title\\\":\\\"Ali Fazal\\\",\\\"year\\\":null,\\\"color\\\":null,\\\"content\\\":\\\"<p><span data-sheets-value="[null,2,&quot;I love being in love.&quot;]" data-sheets-userformat="[null,null,14915,[null,0],[null,2,16777215],null,null,null,null,0,null,null,0,null,[null,2,4144959],&quot;georgia&quot;,11]">I love being in love.<\\\\/span><\\\\/p>\\\\n\\\",\\\"birthdate\\\":null,\\\"birthplace\\\":null,\\\"zodiac\\\":\\\"Capricorn\\\"}]"
}
When I tried following commands I get the following errors
db.timeline.find({tdata:{"birthdate":{$exists: true, “$ne”: null }}},{sketchid})
2016-08-19T12:02:14.665+0530 E QUERY [thread1] SyntaxError: illegal character #(shell):1:53
db.timeline.find({tdata:{"birthdate\\":{$exists: true, “$ne”: null }}},{sketchid})
2016-08-19T12:02:53.357+0530 E QUERY [thread1] SyntaxError: unterminated string literal #(shell):1:25
db.timeline.find({tdata:{\\"birthdate\\":{$exists: true, “$ne”: null }}},{sketchid})
2016-08-19T12:03:06.572+0530 E QUERY [thread1] SyntaxError: illegal character #(shell):1:25

Answer 1

You need to use the positional operator if you want to test only the first element of the tdata array. Also depending on your version {$ne:null} doesn't always give what you'd expect. You can check against the null type 10 instead:
db.timeline.find({"tdata.0.birthdate": {$exists:true,$not : { $type : 10 }} },{sketchid:1})