MongoDb : Basic Commands

Before we actually get into MongoDb, let’s begin by importing a Test Db. You can access the JSON file here. Once you have downloaded the file, use following command to import the document into mongodb.

mongoimport --db testdb --collection students --drop --students.json

Let’s break it down, the command tells you to import a file students.json into a database called testdb and collection called students. Let’s go ahead verify if our import is good.

show dbs
use students
show collections

We first checked if the db named students has been created by checking the list of dbs. ‘show dbs’ command displays list of all dbs in the server. We followed it up with ‘use students’ command, which is similiar to the ‘use ‘ command in MySql. Finally, we issued ‘show collection’ command to view the collections in the db. Remember collections are analogous to tables in relational database.

We will now go ahead check out data. In relational database, data is saved as a row in the table. When it comes to Nosql databases, each row is represented as a document.
To view contents of a collection, we issue a find command
db..find()
In our case, it would be

db.students.find()

This would show our 3 documents within the collection. Let’s explore the find command a bit. We could give a specific search filter as arguement, in case we need to find a particular document, similiar to the ‘where’ condition in relational database. Instead of the customary “=” syntax, MongoDb relies on JSON for passing the argument.

db.students.find({"name":"jia anu"})

In case you want to pass more than one arguement, things don’t change much. Yes, you guess right, you pass more data in json.

db.students.find({"name":"jia anu","age":1})

If we need to search use a comparison operator ‘greater than’ or ‘less than’, you would need to use the ‘$gte and $lte commands.
Let’s go ahead and issue a command, say, we need to find all students who age is ‘less than or equal to 1’

db.students.find({"name":"jia anu","age":{$lte:1}})

A comprehensive list of comparison operator can be found in here

The complete list of beginners guide in this series can be found in here

Workaround for MySql 5.7 EF DbFirst Issue

Anyone working on .Net application with MySql 5.7 and EF in Db First Approach would have come across what is an officially confirmed bug.  This bug rises when you attempt to generate/update your Entity Model after changes to the Database.

“Unable to generate the model because of the following exception: ‘System.Data.StrongTypingException: The value for column ‘IsPrimaryKey’ in table ‘TableDetails’ is DBNull. —> System.InvalidCastException:”

The workaround you need to do to fix this one

  1. Open your Workbench and execute following commands on your Db.
set global optimizer_switch='derived_merge=off';
set optimizer_switch='derived_merge=off';
  1. Close your workbench ( you might have to restart your visual studio as well ).
  2. Generate/Update your model.
That should work.