In Elasticsearch, how to move data from one field into another field - elasticsearch

Question

I have an index with mappings that look like this:
"mappings": {
"default": {
"_all": {
"enabled": false
},
"properties": {
"Foo": {
"properties": {
"Bar": {
"type": "keyword"
}
}
}
}
}
I am trying to change the mapping to introduce a sub-field of Bar, called Code, whilst migrating the string currently in Bar into Bar.Code. Here is the new mapping:
"mappings": {
"default": {
"_all": {
"enabled": false
},
"properties": {
"Foo": {
"properties": {
"Bar": {
"properties": {
"Code": {
"type": "keyword"
}
}
}
}
}
}
}
In order to do this, I think I need to do a _reindex and specify a pipeline. Is that correct? If so, how does my pipeline access the original data?
I have tried variations on the following code, but without success:
PUT _ingest/pipeline/transformFooBar
{
"processors": [
{
"set": {
"field": "Bar.Code",
"value": "{{_source.Bar}}"
}
}
]
}
POST _reindex
{
"source": {
"index": "foo_v1"
},
"dest": {
"index": "foo_v2",
"pipeline": "transformFooBar"
}
}

Answer 1

Ah, I almost had the syntax right. The _source is not required:
// Create a pipeline with a SET processor
PUT _ingest/pipeline/transformFooBar
{
"processors": [
{
"set": {
"field": "Bar.Code",
"value": "{{Bar}}"
}
}
]
}
// Reindex using the above pipeline
POST _reindex
{
"source": {
"index": "foo_v1"
},
"dest": {
"index": "foo_v2",
"pipeline": "transformFooBar"
}
}