Querying MongoDB

This is a WIP.

Basic listing of Elements

You can list your collections by entering

show collections

in order to list all elements in a collection type

db.collectionname.find()

for example, to list all elements in livetraindata, use db.livetraindata.find().

Filtering data

Expressions

In MongoDB, different things like operators, comparisons are written as expressions. each expression is surrounded by curly brackets:

{ expression }

these expressions can then be put into functions, for example if we want to filter for a firstTrainID "4050", we would use the following script:

db.getCollection('livetraindata').find({firstTrainNumber: 49050})

There are many different operators like $and, $or, $match (which is the SQL-where)

If we want to query for two possible trainIds, we would write it like this:

db.getCollection('livetraindata').find(
{$or: [
    {firstTrainNumber: 49050},
    {firstTrainNumber: 444310}
    ]}
)

Operators enclose their operands with [square brackets]. These can then hold as many operands as you like.

complex aggregations

MongoDB also supports SQL-like queries. For this, use the .aggregate([]) function on the collection. An aggregations is done in a pipeline, each operand passing its documents to the next part. The operands can be defined inside the square brackets. Consider the following example:

db.getCollection('livetraindata').aggregate([
    {$match: 
        {firstTrainNumber: 49050}        
    },
    {$lookup : 
        {
        localField: "locationId",
        from: "locationdata",
        foreignField: "id",
        as: "locationdata"
        }
    },

    {$match : 
            {$or: [
                {"locationdata.country":"DE"},
                {"locationdata.country":"AT"},
                {"locationdata.country":"CH"}
            ]}
    }
]);

This script would first filter each entry for its firstTrainNumber ($match), join it with the locations collection on the locationId (&lookup), and lastly filter once again to only include specific countries.