setindex! introducing a constraint isn't always desirable

[glennmoy]

This came up in an internal implementation.

When we introduce a new element to a KeyedDataset the setindex! method assigns a constraint if one isn't already associated with it's dimpath.

AxisSets.jl/src/indexing.jl

Lines 122 to 130 in eb33c8c

	for d in dimnames(val)
	dimpath = (key..., d)
	# Similar to construction, if our dimpath isn't present in any existing constraints
	# then we introduce a new one that's `Pattern(:__, dim)`
	if all(c -> !in(dimpath, c), ds.constraints)
	push!(ds.constraints, Pattern(:__, d))
	end
	end

However, this isn't necessarily desirable as the new element might share a fieldname with an existing element and introduce and so introduce a conflicting constraint.

MWE

julia> using AxisSets, AxisKeys
julia> using AxisSets: Pattern

# assume the following
julia> ds = KeyedDataset(
           (:train, :input) => KeyedArray(ones(5, 5); time=1:5, id='a':'e'),
           (:predict, :input) => KeyedArray(ones(5); id='a':'e'),
           (:train, :output) => KeyedArray(ones(5); time=1:5);
           constraints=Pattern[
               (:train, :_, :time),
               (:predict, :_, :time),
               (:__, :input, :id),  # offending constraint
           ]
       )

# assign a new element with `:id` dimname - introduces `(:__, :id)` constraint
julia> ds[(:train, :weights)] = KeyedArray(ones(5), id='a':'e')

julia> ds.constraints
OrderedCollections.OrderedSet{Pattern} with 4 elements:
  Pattern((:train, :_, :time))
  Pattern((:predict, :_, :time))
  Pattern((:train, :input, :id))
  Pattern((:__, :id))

julia> ds
Error showing value of type KeyedDataset:
ERROR: ArgumentError: Collection has multiple elements, must contain exactly 1 element
Stacktrace:
  [1] only
    @ ./iterators.jl:1327 [inlined]
  [2] _only(x::Vector{Int64})
    @ AxisSets ~/.julia/packages/AxisSets/27klG/src/AxisSets.jl:27
  [3] (::AxisSets.var"#20#22"{Vector{Pattern}, Tuple{Symbol, Symbol}})(dimname::Symbol)
    @ AxisSets ~/.julia/packages/AxisSets/27klG/src/dataset.jl:97
  [4] map(f::AxisSets.var"#20#22"{Vector{Pattern}, Tuple{Symbol, Symbol}}, t::Tuple{Symbol, Symbol})
    @ Base ./tuple.jl:214
  [5] show(io::IOContext{Base.TTY}, ds::KeyedDataset)
    @ AxisSets ~/.julia/packages/AxisSets/27klG/src/dataset.jl:95
   ...

In this instance one would be better off redefining the offending constraint as (:__, :id) but this might not always be possible/desirable.