Query one document or another depending on specific field - solr

Question

Imagine a SolR-index with documents similar to this
[
{
ProductId: 123,
Contract: abc
},
{
ProductId: 123,
Contract: def
},
{
ProductId: 123
},
{
ProductId: 567
},
{
ProductId: 567,
Contract: bar
}
]
There is always a document with a specific ProductId and without a Contract
Additionally there may be 0 to n documents with Contract
I need a query, where I can use a Contract and that should return me all ProductIds either the one with the given Contract, if exists, or the single document without a Contract at all.
For example I will make a query with Contract: def (somehow) and it should give me this
[
{
ProductId: 123,
Contract: def
},
{
ProductId: 567
}
]
The document with Contract:abc is not part of the result
The document with ProductId:123 but without Contract is not part of the result
The document ProductId:567 is part of the result, because there is no document with this ProductId and ContractId: def
In other words what I need is something like
Give me one documents per ProductId and with Contract:X XOR -Contract*, but not both.

Answer 1

Step 1 Write your query so that records without Contracts as well as all with matching contracts are returned, but the ones with the appropriate contract have the highest score. This gets around the problem that you will sometimes want items in your results that don't match the contract value: q=Contract:"def" OR (*:* -Contract:[* TO *]). The (*:* -Contract:[* TO *]) matches that all records without contracts, and the Contract:"def" matches records with the correct contract. The records matching Contract:"def" should naturally have a higher score than those with no contract, but if there's any trouble or you just want to be sure, you can add a boost to that clause, Contract:"def"^2.
Step 2 Add Result Grouping to the query, configured so that you are requesting only the highest scoring record for any given ProductId:
q=Contract:"def" OR (*:* -Contract:[* TO *])&group=true&group.field=ProductId
This requires that the ProductId field be configured in your schema.xml as multiValued="false", as multiValued fields cannot be used as groups. I'm also assuming that you are using the Standard Query Parser, either set as a default in your solrconfig.xml or by adding the argument defType=lucene when you make the query.
The results should look something like this:
'grouped'=>{
'ProductId'=>{
'matches'=>5,
'groups'=>[{
'groupValue'=>123,
'doclist'=>{'numFound'=>3,'start'=>0,'docs'=>[
{
'ProductId'=>123,
'Contract'=>'def'}]
}},
{
'groupValue'=>567,
'doclist'=>{'numFound'=>2,'start'=>0,'docs'=>[
{
'ProductId'=>567}]
}}]}}}
Note that neither the matches nor the numFound values in the result set will tell you how many groups have been returned, but the argument rows=XX can be used to define the maximum number of desired groups (in this case ProductIds).