Search query for elastic search - elasticsearch

Question

I have documents in elastic search in the following format
{
"stringindex" : {
"mappings" : {
"files" : {
"properties" : {
"BaseOfCode" : {
"type" : "long"
},
"BaseOfData" : {
"type" : "long"
},
"Characteristics" : {
"type" : "long"
},
"FileType" : {
"type" : "long"
},
"Id" : {
"type" : "string"
},
"Strings" : {
"properties" : {
"FileOffset" : {
"type" : "long"
},
"RO_BaseOfCode" : {
"type" : "long"
},
"SectionName" : {
"type" : "string"
},
"SectionOffset" : {
"type" : "long"
},
"String" : {
"type" : "string"
}
}
},
"SubSystem" : {
"type" : "long"
}
}
}
}
}
}
My requirement is when I search for a particular string (String.string) i want to get only the FileOffSet (String.FileOffSet) for that string.
How do i do this?
Thanks

Answer 1

I suppose that you want to perform a nested query and retrieve only one field as the result, but I see problems in your mapping, hence I will split my answer in 3 sections:
What is the problem I see:
How to query nested fields (this is more ES background):
How to find a solution:
1) What is the problem I see:
You want to query a nested field, but you don't have a nested field.
The nested field part:
The field "Strings" is not nested in the type "files" (nested data without a nested field may bring future problems), otherwise your mapping for the field "Strings" would be something like this:
{
"stringindex" : {
"mappings" : {
"files" : {
"properties" : {
"Strings" : {
"properties" : {
"type" : "nested",
"String" : {
"type" : "string"
}
}
}
}
}
}
}
}
Note: yes, I cut most of the fields, but I did this to easily show that you didn't create a nested field.
With a nested field "in hands", we need a nested query.
The specific field result part:
To retrieve only one field as result, you have to include the property "_source" in your query.
2) How to query nested fields:
This is more for ES background, if you have never worked with nested fields.
Small example:
You define a type with a nested field:
{
"nesttype" : {
"properties" : {
"name" : { "type" : "string" },
"parents" : {
"type" : "nested" ,
"properties" : {
"sex" : { "type" : "string" },
"name" : { "type" : "string" }
}
}
}
}
}
You create some inputs:
{ "name" : "Dan", "parents" : [{ "name" : "John" , "sex" : "m" },
{ "name" : "Anna" , "sex" : "f" }] }
{ "name" : "Lana", "parents" : [{ "name" : "Maria" , "sex" : "f" }] }
Then you query, but only fetch the nested field "parents.name":
{
"query": {
"nested": {
"path": "parents",
"query": {
"bool": {
"must": [
{
"term": {
"sex": "m"
}
}
]
}
}
}
},
"_source" : [ "parents.name" ]
}
The output of this query is "the name of the parents of all people who have a parent of the sex 'm' ". One entry (Dan) has a father, whereas the other (Lana) doesn't. So it only will retrieve Dan's parents names.
3) How to find a solution:
To fix your mapping:
You only need to include the type "nested" in the field "Strings":
{
"files" : {
"properties" : {
...
"Strings" : {
"type" : "nested" ,
"properties" : {
"FileOffset" : { "type" : "long" },
"RO_BaseOfCode" : { "type" : "long" },
...
}
}
...
}
}
}
To query your data:
{
"query": {
"nested": {
"path": "Strings",
"query": {
"bool": {
"must": [
{
"term": {
"String": "my string"
}
}
]
}
}
}
},
"_source" : [ "Strings.FileOffSet" ]
}

Answer 2

Great answer by dan, but I think he didn't mention it all.
His solution don't work for your question, but I guess you even don't know that.
Consider a scenario where data is like ,
doc_1
{
"Id": 1,
"Strings": [
{
"string": "x",
"fileoffset": "f1"
},
{
"string": "y",
"fileoffset": "f2"
}
]
}
doc_2
{
"Id": 2,
"Strings": {
"string": "z",
"fileoffset": "f3"
}
}
When you run the like dan said, like say let's apply filter with Strings.string=x then response is like,
{
"hits": [
{
"_index": "stringindex",
"_type": "files",
"_id": "11961",
"_score": 1,
"_source": {
"Strings": [
{
"fileoffset": "f1"
},
{
"fileoffset": "f2"
}
]
}
}
]
}
This is because, elasticsearch will get hits from documents where any of the object inside nested field (here Strings) pass the filter criteria. (In this case in doc_1, Strings.string=x passed filter, so doc_1 is returned. But we don't know which nested object pass the criteria.
So, you have to use nested_aggregation,
Here is a solution for you..
POST index/type/_search
{
"size": 0,
"aggs": {
"StringsNested": {
"nested": {
"path": "Strings"
},
"aggs": {
"StringFilter": {
"filter": {
"term": {
"Strings.string": "x"
}
},
"aggs": {
"FileOffsets": {
"terms": {
"field": "Strings.fileoffset"
}
}
}
}
}
}
}
}
So, response is like,
"aggregations": {
"StringsNested": {
"doc_count": 2,
"StringFilter": {
"doc_count": 1,
"FileOffsets": {
"buckets": [
{
"key": "f1",
"doc_count": 1
}
]
}
}
}
}
Remember to have mapping of Strings as nested, as dan said.