MongoDB QueryNestedData

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Querying a collection

The find() function can be used as before to query the documents:

 db.collectionName.find(optional_query_criteria)

Remember:

the criteria is enclosed in curly brackets: {}
the value needs quotes if it is a string or date value
all names and values are case sensitive
quotes are optional for the key field, so long as they do not contain spaces
if the key field refers to a nested document, the name must be in matching single or double quotes

For example, show all the data so far in the deptCollection:

db.deptCollection.find()

The data comes back messy. The pretty() function can be used to improve the layout::

db.deptCollection.find().pretty()

To find just one document - department 10:

db.deptCollection.find({deptno:10}).pretty()

Finding an employee means using the array name too:

db.deptCollection.find({"employees.empno":7902}).pretty()

However, this does mean you get back all the employees in the department they were found in!

Since Version 2.2 MongoDB's new $elemMatch can be used with arrays to return only the first element matching the $elemMatch condition:

db.deptCollection.find({deptno:20},  
  { _id: 0, employees: {$elemMatch: {empno: 7902}}}).pretty()

$elemMatch limits the contents of the employees array to contain only the first element matching the $elemMatch condition.

This is akin to a SQL query:

 SELECT * FROM Emp WHERE deptno=20 AND empno = 7902

_id is a unique value automatically generated by MongoDB (like a Primary Key, except it is unique for the whole database).

Using _id:0 suppresses the ObjectId value, however to see it:

db.deptCollection.find({deptno:20},  
  { employees: {$elemMatch: {empno: 7902}}}).pretty()

More about _ids in the next section.

The query criteria can be as complex as that found in SQL.

To find all employees earning more than 2000 in department 10:

db.deptCollection.find({deptno:10},   
 { employees: {$elemMatch: {sal: { $gt: 2000}}}}).pretty()

Same again for department 20 and the managers:

db.deptCollection.find({deptno:20},  { employees: {$elemMatch: {sal: { $gt: 2000}, job: "MANAGER"}}}).pretty()

employees is an array, so $elemMatch only returns the first matching value. What if we try this instead:

db.deptCollection.find({ "employees.sal" : { $gt: 2000}}).pretty()

Things to note:

This time employees.sal must be enclosed in matching single or double quotes.
Comment on what the above query returns.
If you examine the data carefully:
- either find() means if one element of an array is found to be true, then all the elements are returned, even if they do not satisfy the query criteria
- or including $elemMatch means only the first element matching the query criteria is returned.
Is this good practice?

Find departments with no managers:

db.deptCollection.find({ "employees.job" : { $ne: "MANAGER"}}).pretty()

MongoDB Aggregation Pipeline and nested data