Project Mongo collection array value under a condition - php

Question

i am using the Mongo php library in a service that is responsible of storing and retrieving some social data, specifically from Facebook, and just a snippet, it goes something like this, a collection of posts insights:
{
"_id":"5865aa8e9bbbe400010f97a2",
"insights":[
{
"name":"post_story_adds_unique",
"period":"lifetime",
"values":[
{
"value":10
}
]
},
{
"name":"post_story_adds",
"period":"lifetime",
"values":[
{
"value":11
}
]
},
{
"name":"post_story_adds_by_action_type_unique",
"period":"lifetime",
"values":[
{
"value":{
"like":10,
"comment":1
}
}
]
},
{
"name":"post_story_adds_by_action_type",
"period":"lifetime",
"values":[
{
"value":{
"like":10,
"comment":1
}
}
]
},
{
"name":"post_impressions_unique",
"period":"lifetime",
"values":[
{
"value":756
}
]
}
]
},
{
"_id":"586d939b9bbbe400010f9f12",
"insights":[
{
"name":"post_story_adds_unique",
"period":"lifetime",
"values":[
{
"value":76
}
]
},
{
"name":"post_story_adds",
"period":"lifetime",
"values":[
{
"value":85
}
]
},
{
"name":"post_story_adds_by_action_type_unique",
"period":"lifetime",
"values":[
{
"value":{
"like":73,
"comment":8,
"share":2
}
}
]
},
{
"name":"post_story_adds_by_action_type",
"period":"lifetime",
"values":[
{
"value":{
"like":74,
"comment":9,
"share":2
}
}
]
},
{
"name":"post_impressions_unique",
"period":"lifetime",
"values":[
{
"value":9162
}
]
}
]
}
We can note that all the posts' sub-documents, i.e. metrics, are present in each post, surely, with their corresponding values.
I believe that the data structure presented above is not optimal to work with; if faced with the situation to project a couple of these sub-documents values, say post_story_adds_by_action_type's and post_impressions_unique's value property, we need to run a condition on all the sub-documents in order to match the name property to post_story_adds_by_action_type or post_impressions_unique.
I tried $elemMatch(projection), but as the documentation says, it only returns the first matching array element. I was able to do so as such:
$this->db->$collection->find([],
[
'projection' => [
'insights' => [
'$elemMatch' => [
'name' => 'post_impressions_unique'
]
],
'_id' => 0,
'insights.values.value' => 1,
],
]);
A MongoDB\Driver\Cursor object is returned with only the desired value, by setting this 'insights.values.value' => 1, of the sub-documents that have name equal to post_impressions_unique.
The first thing i thought about was, of course, to use the $or logical operator:
$this->db->$collection->find([],
[
'projection' => [
'insights' => [
'$elemMatch' => [
'$or' => [
[
'name' => [
'$eq' => 'post_story_adds_by_action_type',
]
],
[
'name' => [
'$eq' => 'post_impressions_unique',
]
]
]
]
]
],
'_id' => 0,
'insights.name' => 1,
'insights.values.value.like' => 1,
'insights.values.value' => 1,
]);
Note the projections 'insights.values.value.share' => 1 'insights.values.value' => 1 corresponding to the different sub-documents value position.
Of course this didn't work, i got an array of post_impressions_unique sub-documents alone; so i had to try the aggregation framework https://docs.mongodb.com/manual/reference/operator/aggregation/filter/ :
$this->db->$name->aggregate(
[
[
'$project' => [
'insights' => [
'$filter' => [
'input' => '$insights',
'as' => 'name',
'cond' => [
'$or' => [
[
'$eq' => [
'name', '$post_story_adds_by_action_type',
]
],
[
'$eq' => [
'name', '$post_impressions_unique',
]
],
],
],
],
],
'_id' => 0,
'values.value.like' => 1,
'values.value' => 1,
],
]
]);
This didn't work either, in this case i got an array of empty insights objects.
I considered using the laravel-mongodb package and take advantage of the Eloquent builder.
DB::collection($collection)->project(['insights' => ['$elemMatch' => ['name' => 'post_story_adds_unique']]])->get();
or
DB::collection($collection)->project(['insights' => [
'$filter' => [
'input' => '$insights',
'as' => 'name',
'cond' => [
'name' => 'post_story_adds_unique',
]
]
],
'values.value.like' => 1
])->get();
But still i couldn't get the value within the sub-document. I checked the Builder::project() function and it seems that it internally use the aggregation framework as well, but i wasn't able to figure out the appropriate syntax to do so, if any.
My questions are as follow:
1- How can i retrieve specific sub-documents' insights.values.value and name properties, where name matches post_impressions_unique? An vice versa, how to retrieve the sub-documents' insignts.values.value.share and name properties when the latter matches post_story_adds_by_action_type?
2- What is the correct syntax to use $filter within an $project(aggregation)?
This is basically most of the research i have been doing, and it feels as if i am running in circles.
Appreciate your help.
Thank you.

Answer 1

You can try something like this.
Use $$ notation to access the variable defined in iteration in $filter and $map.
$map is used to trim the response to display values and name from insights array.
[
[
'$project' => [
'insights' => [
'$map' => [
'input' => [
'$filter' => [
'input' => '$insights',
'as' => 'insightf',
'cond' => [
'$or' => [
[
'$eq' => [
'$$insightf.name', 'post_story_adds_by_action_type',
]
],
[
'$eq' => [
'$$insightf.name', 'post_impressions_unique',
]
],
],
],
],
],
'as' => 'insightm',
'in' => [
'values' => '$$insightm.values.value',
'name' => '$$insightm.name',
],
],
],
'_id' => 0
],
]
];