[GIS] Unable to calculate values in a joined table

arcgis-10.4arcgis-desktopattribute-joinsfield-calculator

Using Arc 10.4.1, I have joined a geodatabase feature class table to a standalone geodatabase attribute table. This is a many:1 join (standalone attribute table:feature class). After making the join, I need to do a series of queries on the standalone attribute table, then calculate a field in the feature class table. Because of this many:1 situation, I am forced to join the feature class table to the standalone attribute table, rather than joining the standalone attribute table to the feature class table.

However, when I try to change values in the joined (feature class) table, the field calculator is greyed out. Sure enough, the Arc Help documentation states:

"You can only calculate values for fields in the origin table."

But I could swear that I've successfully used the field calculator on a joined table in the past (Arc 9.x, methinks).

Is there a way to create calculations on joined fields?

Best Answer

As commented by @tsvgis:

Start editing the table and then calculate the fields. I think that has worked for me in the past. Additionally, before the join, make a new field for the calculations, then joins and do the calculations in the new field

Related Solutions

[GIS] How to Calculate Field in joined table using ArcPy

Read the help for Add Join. In particular:

"The input must be a feature layer, a table view, or a raster layer that has an attribute table; it cannot be a feature class or table."

Of course, you are passing a table, but a join does not alter a table (i.e. the database), only a layer or a table view (i.e. in memory). That means by the time you call CalculateField_management, Table_A--a table or feature class--still does not have any more columns than it had before.

I think your best solution is to do what the sample in the help does: call MakeFeatureLayer_management first. If Table_A is just a table and not a feature class, call MakeTableView_management instead.

arcpy.MakeTableView_management(Table_A, "table_a_view")
arcpy.AddJoin_management("table_a_view", "id_grid", Table_B, "id_matrix")
arcpy.CalculateField_management(Table_A, Table_A + ".field_A", "!" + Table_B + ".field_B!","PYTHON")

Then "table_a_view" will be a table view, which means the join should work.

[GIS] Updating a feature class

How about something like this:

import arcpy
mainTbl = r"C:\test.gdb\main"
lutTbl = r"C:\test.gdb\lut"
#Note the order of the fields in the search cursor has to coincide with field order in the update cursor
#Also assumes the matching fields are of compatibale types (exanple: can't insert string into integer)
lutDict = {r[0]:[i for i in r[1:]] for r in arcpy.da.SearchCursor(lutTbl, ["JOINFIELD","FIELD_A","FIELD_B","FIELD_C"])}
updateRows = arcpy.da.UpdateCursor(mainTbl, ["JOINFIELD","FIELD_A","FIELD_B","FIELD_C"])
for updateRow in updateRows:
    joinFieldVal = updateRow[0] 
    updateRow[1:] = lutDict[joinFieldVal]
    updateRows.updateRow(updateRow)
del updateRow, updateRows, lutDict

Best Answer

Related Solutions

[GIS] How to Calculate Field in joined table using ArcPy

[GIS] Updating a feature class

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