Last Sync: 2022-08-11 13:30:05

Databases/MongoDB/Querying_a_collection_with_Mongoose.md

@@ -45,7 +45,7 @@ Now we will query the collection. This capability is provided via the Mongoose s
 
 The various `find` methods return a value that is promisified.
 
-## Return all values in a collection: `find`
+## Return values with `find`
 
 ```js
 async function getCourses(){
@@ -54,7 +54,9 @@ async function getCourses(){
 }
 ```
 
-### Filter values returned
+## Filter values returned
+
+This will return all the published courses where Tosh Gnomay is the author:
 
 ```js
 async function getCourses(){
@@ -62,3 +64,61 @@ async function getCourses(){
   console.log(courses)
 }
 ```
+
+This time we will filter by the same author but we will add additional parameters to distinguish:
+
+* only the first ten entries (using `.limit(10)`)
+* sort ascending by name (using `.sort({name: 1}))` , to descend we would use `-1`) 
+* only return the properties `name` and `tags` for the item in the collection (using `.select({name: 1, tags: 1})`)
+
+```js 
+async function getCourses(){
+  const courses = await Course
+  .find({author: 'Tosh Gnomay', isPublished: true})
+  .limit(10)
+  .sort({name: 1})
+  .select({name: 1, tags: 1})
+  console.log(courses)
+}
+
+```
+
+This returns: 
+
+```js
+[
+  {
+    _id: new ObjectId("62f4f07a875cff48827b8731"),
+    name: 'Java Course',
+    tags: [ 'java', 'backend' ]
+  },
+  {
+    _id: new ObjectId("62f4e2527ac4aa2c30d41d24"),
+    name: 'Javascript Course',
+    tags: [ 'js', 'frontend' ]
+  }
+]
+```
+
+> Note that the UUID is always returned, whether we specify it or not.
+
+## Comparison operators
+
+The following comparison operators are available in MongoDB:
+
+| Operator | Function                  |
+|----------|---------------------------|
+| `eq`     | Equal to                  |
+| `ne`     | Not equal to              |
+| `gt`     | Greater than              |
+| `gte`    | Greater than or less than |
+| `lt`     | Less than                 |
+| `lte`    | Less than or equal to     |
+| `in`     | In                        |
+| `nin`    | Not in                    |
+
+We can employ these comparators within a `.find` filter. For example let's imagine that our `courses` instances have a property of `price`. To filter prices $>= 10 \And \And <= 20$:
+
+```js
+Course.find(({price: {$gte: 10, $lte: 20} }))
+```