Now you can run left-outer equi-joins query using $lookup operator in the MongoDB Aggregation Framework. With the help of Joins you can improve your application’s performance by reducing the number of queries on MongoDB.<\/p>\n
What is a Left Outer Equi-Join?<\/strong><\/p>\n Left Outer Equi-Join will have all data from left collection and only matching data from right collection.<\/p>\n $lookup operator performs a left outer join to an unsharded collection in the same database to filter in documents from the \u201cjoined\u201d collection for processing.<\/p>\n The $lookup operator has the following syntax:<\/p>\n [sourcecode language=”css”] [\/sourcecode]<\/p>\n Let me explain join with an example.<\/p>\n Orders collection:<\/strong><\/p>\n [sourcecode language=”css”] [\/sourcecode]<\/p>\n Products collection:<\/strong><\/p>\n [sourcecode language=”css”] { [\/sourcecode]<\/p>\n $lookup can be used in conjunction with other pipeline operators in aggregation framework to make a lookup into cross collection ( i,e left outer joins).<\/p>\n![\"\"](\"\/blog\/wp-ttn-blog\/uploads\/2024\/01\/image00-9a0e627f57.png\")
<\/p>\n
\n{
\n $lookup:
\n {
\n from: <collection to join>,
\n localField: <field from the input documents>,
\n foreignField: <field from the documents of the "from" collection>,
\n as: <output array field>
\n }
\n}<\/p>\n
\n{
\n "_id" : 2.0,
\n "items" : [
\n 3.0,
\n 4.0
\n ]
\n}<\/p>\n
\n{
\n "_id" : 3.0,
\n "item_name" : "Desi Ghee",
\n "item_price" : 500.0,
\n "quantity" : "2 kg"
\n}<\/p>\n
\n "_id" : 4.0,
\n "item_name" : "Namkeen",
\n "item_price" : 50.0,
\n "quantity" : "1 pkt"
\n}<\/p>\n