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Refactoring Table.ChangeType to share value transformation logic #483

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Refactoring Table.ChangeType to share value transformation logic #483

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38 changes: 6 additions & 32 deletions samples/TripPin/9-TestConnection/Table.ChangeType.pqm
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
let
let
// table should be an actual Table.Type, or a List.Type of Records
Table.ChangeType = (table, tableType as type) as nullable table =>
// we only operate on table types
Expand Down Expand Up @@ -34,36 +34,10 @@

reordered = Table.SelectColumns(_table, schema[Name], MissingField.UseNull),

// process primitive values - this will call Table.TransformColumnTypes
map = (t) => if Type.Is(t, type table) or Type.Is(t, type list) or Type.Is(t, type record) or t = type any then null else t,
mapped = Table.TransformColumns(schema, {"Type", map}),
omitted = Table.SelectRows(mapped, each [Type] <> null),
existingColumns = Table.ColumnNames(reordered),
removeMissing = Table.SelectRows(omitted, each List.Contains(existingColumns, [Name])),
primitiveTransforms = Table.ToRows(removeMissing),
changedPrimitives = Table.TransformColumnTypes(reordered, primitiveTransforms),

// Get the list of transforms we'll use for Record types
recordColumns = Table.SelectRows(schema, each Type.Is([Type], type record)),
recordTypeTransformations = Table.AddColumn(recordColumns, "RecordTransformations", each (r) => Record.ChangeType(r, [Type]), type function),
recordChanges = Table.ToRows(Table.SelectColumns(recordTypeTransformations, {"Name", "RecordTransformations"})),

// Get the list of transforms we'll use for List types
listColumns = Table.SelectRows(schema, each Type.Is([Type], type list)),
listTransforms = Table.AddColumn(listColumns, "ListTransformations", each (t) => List.ChangeType(t, [Type]), Function.Type),
listChanges = Table.ToRows(Table.SelectColumns(listTransforms, {"Name", "ListTransformations"})),

// Get the list of transforms we'll use for Table types
tableColumns = Table.SelectRows(schema, each Type.Is([Type], type table)),
tableTransforms = Table.AddColumn(tableColumns, "TableTransformations", each (t) => @Table.ChangeType(t, [Type]), Function.Type),
tableChanges = Table.ToRows(Table.SelectColumns(tableTransforms, {"Name", "TableTransformations"})),

// Perform all of our transformations
allColumnTransforms = recordChanges & listChanges & tableChanges,
changedRecordTypes = if (List.IsEmpty(allColumnTransforms)) then changedPrimitives else Table.TransformColumns(changedPrimitives, allColumnTransforms, null, MissingField.Ignore),

// set final type
withType = Value.ReplaceType(changedRecordTypes, tableType)

// Perform column transformations & set final type
changedTypes = Table.TransformColumns(reordered, GetTransformsForType(tableType)),
withType = Value.ReplaceType(changedTypes, tableType)
in
if (List.IsEmpty(Record.FieldNames(columnsForType))) then table else withType meta previousMeta,

Expand Down Expand Up @@ -136,4 +110,4 @@
else if (Type.Is(_type, type list)) then (t) => if (t <> null) then @List.ChangeType(t, _type) else t
else (t) => t
in
Table.ChangeType
Table.ChangeType