View Source ActiveMemory.Query (ActiveMemory v0.8.1)
The match/1 macro, for queries that need more than equality.
An attributes map (Store.select(%{department: "sales"})) matches fields for
equality. When you need comparisons or boolean logic, match/1 compiles an
Elixir expression into an ETS/Mnesia match specification at compile time.
import ActiveMemory.Query
query = match(:department == "sales" or :department == "marketing" and :start_date > last_month)
Store.select(query)Link to this section Summary
Functions
Build a match query from an expression.
Link to this section Functions
Build a match query from an expression.
Field names are written as atoms — :age, not person.age — and compared against
values. The result is passed to one/1, select/2 or withdraw/1.
operators
Operators
Comparison: ==, !=, ===, !==, <, <=, >, >=.
Boolean: and, or, not.
match(:age > 30)
match(:cylon? == true)
match(:hair_color == "brown" and :age > 45)
match(:department == "sales" or :department == "marketing")
values-must-be-literals-of-a-supported-type-or-variables
Values must be literals of a supported type, or variables
A value written inline must be an integer, binary or atom (booleans and nil
included). Any other literal — a float, a Decimal, a DateTime — raises a
FunctionClauseError while the query is being compiled:
match(:radius_km > 1000.0) # ** (FunctionClauseError) no function clause
# matching in ActiveMemory.Query.reduce/1Bind it to a variable instead. A variable is evaluated at runtime, so any term works:
cutoff = 1000.0
match(:radius_km > cutoff) # {:>, :radius_km, 1000.0}
yesterday = DateTime.add(DateTime.utc_now(), -1, :day)
match(:inserted_at > yesterday)
comparisons-use-erlang-term-order
Comparisons use Erlang term order
The generated match spec compares with Erlang's term ordering, which is correct for
numbers, binaries and atoms but not for struct based values. A Decimal,
DateTime or NaiveDateTime is a map, and maps sort above every number and compare
field by field, so match(:gravity > threshold) on a :decimal field will not do
what you expect. Filter those in Elixir after reading, or store a comparable
representation (an integer of milliseconds, for instance) alongside.
Ordering results has the same caveat handled for you — see the :order_by option
documented on ActiveMemory.Store, which does use a value's own compare/2.
what-it-does-not-support
What it does not support
There are no joins, aggregates, subqueries, select projections or group_by. A
match query filters records of one table and returns whole structs. For anything
else, read and work in Elixir.