Today lets see how we can track the index number of the loop. i.e., the number of completed iterations in a loop. We shall also learn *ngIf conditional operator and its usage.
ngIf is called conditional operator because it operates based on conditions i.e., if the conditional statement it has been assigned is true, then the node it is attached to will be rendered or else it’ll not. *ngIf takes boolean values.
Example: If *ngIf is attached to a div and the condition is false, then the div it has been attached to won’t be added to the DOM. If the condition is true, then div is added to the DOM.
index, first, last While we are looping through some array elements inside template file we can know the index of the loop using index variable. We can use ngIf and do something at the first iteration using first variable and similarly we can know the last iteration using last variable.
<ion-list no-lines>
<ion-item *ngFor="let company of companies;
let i = index;
let lst = last;
let fst = first;">
{{i+1}}. {{company.name}}
<span *ngIf="lst"> - Am last!</span>
<span *ngIf="fst"> - Am first</span>
</ion-item>
</ion-list>
inside *ngFor we have initiated and assigned index value to variable i, last index value to variable lst and first index value to variable fst.
Inside ion-item we are using *ngIf to check if the iteration is first iteration, if so add ‘Am first’ message besides the first list item i.e., beside Microsoft. We also check if the iteration is a last iteration, if so we display ‘Am last!’ message beside the last item in the list i.e., beside IBM.
We’ve seen the working of explain() method briefly in previous video tutorials – in this video tutorial lets dive in to learn more about this useful method, which helps us analyze and optimize our MongoDB commands.
MongoDB shell version: 2.6.1
connecting to: test
> use temp
switched to db temp
> for(i = 1; i < = 1000; i++) db.name.insert({a: i, b: i, c: i});
WriteResult({ "nInserted" : 1 })
MongoDB shell version: 2.6.1
connecting to: test
> use temp
switched to db temp
> for(i = 1; i < = 1000; i++) db.name.insert({a: i, b: i, c: i});
WriteResult({ "nInserted" : 1 })
Here MongoDB server is returning a basic cursor, as we do not have index on field “a”. Also note that, it’s scanning 1000 documents in the “name” collection and scanning 1000 index keys in “system.indexes” collection, where the default key is “_id”.
Here we create index on key “a” and “b” – it’s called compound key – after which the explain() method shows that MongoDB server is returning a Btree cursor, meaning it’s making use of index key to execute the query/command.
Also not the number of objects scanned in the “name” collection as well as in the “system.indexes” collection.
Covered Index or indexOnly If our command is requesting for values present inside the index, mongo engine will fetch that information from the “system.indexes” collection itself and does not go to the collection where the entire document is present – in our case “name” collection. Since index is on keys “a” and “b” in our case, if we query for “a” and “b” values only, it fetches those values directly from the index information and will not go to “name” collection. Hence the speed of execution of the command is faster, and this type of commands are very efficient in creating optimized web applications.
Note from above explain() result, the number of objects scanned in the “name” collection is 0, and the number of scans made on indexes in only 1.
We learnt how to create unique key/index using {unique: true} option with ensureIndex() method. Now lets see how we can create unique key when there are duplicate entries/documents already present inside the collection.
Now append explain() method to our command, it shows us that it returns a Btree Cursor and multi-key as true. MongoDB engine need to match every element of the array present in field “a” with the scalar value of field “b”. Hence it uses Multi-Key indexing.
It’s difficult to match every combination of the array elements present inside both “a” and “b” fields. If both keys/indexes has its value as an array, then it gets complicated. Thus, mongoDB doesn’t allow both keys to be arrays. Either one of them must be a scalar value.
We learnt the uses of having an index/key on our collection and how to create the index. Now, in this video tutorial lets learn how to get index on individual collection and how to drop / remove / delete the index we’ve created.
After creating “another” collection, mongoDB engine generates default key on its “_id” field. And “system.indexes” shows all the keys present inside the database for all the collections it has. This can get messy if we have large number of collections – which we do in even slightly bigger projects.
