Explaining implicit context in queries in Sources

tpch_graph.malloy

import "model.malloy"

/*

The file `model.malloy` (imported above) is defined as follows:

source: lineitem is table('duckdb:../data/lineitem.parquet') { 
  join_one: 
    orders is table('duckdb:../data/orders.parquet') on orders.o_orderkey = l_orderkey
    customer is table('duckdb:../data/customer.parquet') on customer.c_custkey = orders.o_custkey

  measure :   
    _count is count()
    count_customers is count( distinct customer.c_custkey)

  query: customers_bought_something is {  
    aggregate: count_customers
  }
}
*/

// "customer_bought_something" is a query defined explicitly on the `lineitem` source
// It can only be executed in the context of `lineitem`, as follows:

query: lineitem -> customers_bought_something

// The following syntax throws a compilation error,
// since the query is only defined within `lineitem`
query: -> customers_bought_something
query: customers_bought_something

// In Malloy a Source object can contain a `query` as a property
// When you click the "Run" codelens button on the `query` inside a Source,
// e.g., the `customers_bought_something` query in the above `lineitem` Source,
// the Malloy runtime is implicitly translating that to "lineitem -> customers_bought_something"
// so even though it appears in VSCode that you're only referencing a measure from the `Customers` table,
// the implicit context is still the `lineitem` Source.

// In practice, when running Malloy queries, you "import" the model with the joins and measures defined
// then construct queries off of those sources.

// The "Run" codelens on queries inside of Sources is mostly there for convenience during development.
// When executing a Malloy script.