ElasticSearch - Find matches with exact value - database

Question

I'm new working with elastic search and I'm trying to make a simple query work. I'm using elasticseach 2.3. I can't use a newer version I'm limited by another technology I'm using.
Basically I have News stored in the database with a title, a source and a publication date.
The query I'm trying to make should search for all the news that contain a certain keyword, come from some sources A or B and have a publication date in the range given.
So far I have this:
{
"query":{
"bool":{
"must":[
{
"bool":{
"should":[
{
"match":{
"source":"SOURCE_A"
}
},
{
"match":{
"source":"SOURCE_B"
}
},
{
"match":{
"title": "keyword"
}
}
]
}
}
],
"filter":{
"range":{
"publication_date":{
"gte":"DATE_FROM",
"lte":"DATE_TO"
}
}
}
}
}
}
The problem is that if a given source starts exactly the same as another source (for example: "SOURCE" and "SOURCE ABC") they are both included in the result. I would like to match exactly the same source.
Can anyone point me in the right direction?
Thanks!
The index is being created by Django Haystack but given its limitations I need to query the database myself. The index mapping is the following:
{
"myindex": {
"mappings": {
"modelresult": {
"properties": {
"django_ct": {
"type": "string",
"index": "not_analyzed",
"include_in_all": false
},
"django_id": {
"type": "string",
"index": "not_analyzed",
"include_in_all": false
},
"id": {
"type": "string"
},
"publication_date": {
"type": "date",
"format": "strict_date_optional_time||epoch_millis"
},
"source": {
"type": "string",
"analyzer": "ascii_analyser"
},
"summary": {
"type": "string",
"analyzer": "ascii_analyser"
},
"text": {
"type": "string",
"analyzer": "ascii_analyser"
},
"title": {
"type": "string",
"analyzer": "ascii_analyser"
},
"url": {
"type": "string",
"analyzer": "ascii_analyser"
}
}
}
}
}
}