Get a count of documents in an aggregation under specific requirement
<p>In my collection, each document represents a user-generated quiz, and includes an array field for tags, i.e. History, Science, Math, etc. I am trying to get a count of documents associated with each tag.</p>

<p>The below aggregation results in a unique tag list that look like this: <code>{tags:["History", "Science", "Math"]}</code></p>

<pre><code>db.quizzes.aggregate([    
            {$unwind: "$tags"},    
            {$group: {_id:null, tgs: {$addToSet: "$tags"}}},    
            {$project: {_id:0, tags: "$tgs"}},    
        ])
</code></pre>

<p>However, can the above aggregation also get a count of the number of documents that contains each tag? For example if there were 3 History quizzes, 2 Science quizzes, and 5 Math quizzes, the result would look like this: <code>{tags:[{tag: "History", count: 3}, {tag: "Science", count: 2}, {tag: "Math", count:5}]}</code></p>

<p>Thanks in advance for any tips.</p>

<p>Edited to include collection documents:</p>

<pre><code>{
    "_id" : ObjectId("57d8ccd573099cb013b462b5"),
    "title" : "Presidential Trivia",
    "quiz" : "[{\"question\":\"How many presidents were members of the Whig party?\",\"choices\":[\"Two\",\"Three\",\"Four\"],\"correct\":\"2\"},{\"question\":\"Who was the first president to be impeached?\",\"choices\":[\"Warren Harding\",\"Andrew Johnson\",\"Andrew Jackson\"],\"correct\":\"1\"},{\"question\":\"How many presidents died during their presidency?\",\"choices\":[\"Four\",\"Six\",\"Eight\"],\"correct\":\"2\"},{\"question\":\"How many presidents had no party affiliation?\",\"choices\":[\"One\",\"Two\",\"Three\"],\"correct\":\"0\"},{\"question\":\"Who was the only president to serve two non-consecutive terms, making him both the 22nd and 24th president?\",\"choices\":[\"John Quincy Adams\",\"Grover Cleveland\",\"Theodore Roosevelt\"],\"correct\":\"1\"}]",
    "correctArray" : "[\"2\",\"1\",\"2\",\"0\",\"1\"]",
    "author" : "jake2",
    "createTime" : ISODate("2016-09-14T04:06:45.118Z"),
    "likes" : 0,
    "avgScore" : 0,
    "plays" : 3,
    "private" : "0",
    "tags" : [
        "US Presidents",
        "American History",
        "History"
    ]
}
{
    "_id" : ObjectId("57d8d08973099cb013b462b6"),
    "title" : "Finance Quiz",
    "quiz" : "[{\"question\":\"Which of these involves the analysis of of a business's financial statements, often used in stock valuation?\",\"choices\":[\"Fundamental Analysis\",\"Technical Analysis\",\"P/E ratio\"],\"correct\":\"0\"},{\"question\":\"What was the name of the bond purchasing program started by the U.S. Federal Reserve in response to the 2008 financial crisis?\",\"choices\":[\"Stimulus Package\",\"Quantitative Easing\",\"Mercantilism\"],\"correct\":\"1\"},{\"question\":\"Which term describes a debt security issued by a government, company, or other entity?\",\"choices\":[\"Bond\",\"Stock\",\"Mutual fund\"],\"correct\":\"0\"},{\"question\":\"Which of these companies has the largest market capitalization (as of October 2015)?\",\"choices\":[\"Ford Motors\",\"Apple\",\"Bank of America\"],\"correct\":\"1\"},{\"question\":\"Which of these is a measure of the size of an economy?\",\"choices\":[\"Purchasing Power Index\",\"Unemployment Rate\",\"Gross Domestic Product\"],\"correct\":\"2\"}]",
    "correctArray" : "[\"0\",\"1\",\"0\",\"1\",\"2\"]",
    "author" : "jake2",
    "createTime" : ISODate("2016-09-14T04:22:33.756Z"),
    "tags" : [
        "Finance"
    ],
    "likes" : 0,
    "avgScore" : 0,
    "plays" : 10,
    "private" : "0"
}
{
    "_id" : ObjectId("57d8d24073099cb013b462b8"),
    "title" : "Astronomy Pop Quiz",
    "quiz" : "[{\"question\":\"Which of the following are currently (as of November 2015) used by scientists as observational evidence of the existence of dark matter?\",\"choices\":[\"Gravitational Lensing\",\"Specimens of dark matter collected by NASA\",\"Anomalies in planetary orbits\"],\"correct\":\"0\"},{\"question\":\"Which of these emits the most energy?\",\"choices\":[\"Stars\",\"Quasars\",\"Black Holes\"],\"correct\":\"1\"},{\"question\":\"What is it called when light or electromagnetic radiation from an object is increased in wavelength?\",\"choices\":[\"The Jupiter Effect\",\"Redshift\",\"The Observer's Differential\"],\"correct\":\"1\"},{\"question\":\"Who was the first human in space?\",\"choices\":[\"Yuri Gagarin\",\"Alan Shepard\",\"John Glenn\"],\"correct\":\"0\"},{\"question\":\"Which of these is the most dense?\",\"choices\":[\"The Sun\",\"A neutron star\",\"Earth\"],\"correct\":\"1\"}]",
    "correctArray" : "[\"0\",\"1\",\"1\",\"0\",\"1\"]",
    "author" : "Bertram",
    "createTime" : ISODate("2016-09-14T04:29:52.636Z"),
    "tags" : [
        "Astronomy"
    ],
    "likes" : 1,
    "avgScore" : 0,
    "plays" : 5,
    "private" : "0"
}
{
    "_id" : ObjectId("57d8d3c173099cb013b462ba"),
    "title" : "Film Trivia",
    "quiz" : "[{\"question\":\"Who directed The Godfather trilogy?\",\"choices\":[\"John Huston\",\"Francis Ford Coppola\",\"Martin Scorsese\"],\"correct\":\"1\"},{\"question\":\"What year was the first Ocscar awarded?\",\"choices\":[\"1923\",\"1927\",\"1932\"],\"correct\":\"1\"},{\"question\":\"As of 2010, this and Schindler's List (1993) are the only films to win Best Picture, Director and Screenplay at the Golden Globes, BAFTAs and the Oscars.\",\"choices\":[\"Rain Man\",\"Slumdog Millionaire\",\"Titanic\"],\"correct\":\"1\"},{\"question\":\"In Casablanca, why can't Rick return to America?\",\"choices\":[\"He is indebted to the mob.\",\"He was deported.\",\"No reason is given.\"],\"correct\":\"2\"},{\"question\":\"What was the highest-grossing Western of all time?\",\"choices\":[\"Django Unchained\",\"True Grit\",\"Dances with Wolves\"],\"correct\":\"2\"}]",
    "correctArray" : "[\"1\",\"1\",\"1\",\"2\",\"2\"]",
    "author" : "Pappy2",
    "createTime" : ISODate("2016-09-14T04:36:17.950Z"),
    "tags" : [
        "Movies"
    ],
    "likes" : 1,
    "avgScore" : 0,
    "plays" : 8,
    "private" : "0"
}
{
    "_id" : ObjectId("57ea7f67a58303f01a585e55"),
    "title" : "US History Concepts",
    "quiz" : "[{\"question\":\"\",\"choices\":[\"\",\"\",\"\"]}]",
    "correctArray" : "[]",
    "author" : "martha",
    "createTime" : ISODate("2016-09-27T14:17:11.627Z"),
    "tags" : [
        "US History",
        "History"
    ],
    "likes" : 0,
    "avgScore" : 0,
    "plays" : 1,
    "private" : "0"
}
</code></pre>